Preliminary API for 'new' brains AI system for mobs

Would fix PaperMC#10443 if done.

This is a rough proposal for abstracting over the brains AI system for mobs
for plugins to make use of, similar to the Goals API. I don't think it's feasible
to attempt to make the new AI conform to the old interfaces.

At the moment I understand the rough skeleton and arteries of the system, though I'm
still trying to piece together what does what in user-facing terms.
'get the mob to walk somewhere' has been unsuccessful so far.

As far as API stability concerns, as long as the exposed set of tasks isn't too 1:1 with what
Mojang has, then we should be fine (until the next Big Rewrite:tm: whenever that is) considering
the general stability of the system across versions since its introductions.

There are still some unanswered questions in the API design:
- do events make sense to have here? I'd say no considering the amount of lambdas in the Minecraft implementation
  that make any real introspection API unfeasible.
- do we care to expose sensors/make them modifiable? It could probably be done in a followup PR to make custom
  tasks & sensors a thing, as I'm mostly concerned about exposing what exists in Vanilla at the moment.
- is the arrangement of the entrypoints to the new API sensical?

Test plugin is included in PR to ease testing of the new API until it's ready to ship.

paper-api-generator/generated/io/papermc/paper/registry/keys/ActivityKeys.java

@@ -0,0 +1,217 @@
+package io.papermc.paper.registry.keys;
+
+import static net.kyori.adventure.key.Key.key;
+
+import io.papermc.paper.entity.ai.Activity;
+import io.papermc.paper.generated.GeneratedFrom;
+import io.papermc.paper.registry.RegistryKey;
+import io.papermc.paper.registry.TypedKey;
+import net.kyori.adventure.key.Key;
+import org.jetbrains.annotations.ApiStatus;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * Vanilla keys for {@link RegistryKey#ACTIVITY}.
+ *
+ * @apiNote The fields provided here are a direct representation of
+ * what is available from the vanilla game source. They may be
+ * changed (including removals) on any Minecraft version
+ * bump, so cross-version compatibility is not provided on the
+ * same level as it is on most of the other API.
+ */
+@SuppressWarnings({
+        "unused",
+        "SpellCheckingInspection"
+})
+@GeneratedFrom("1.20.4")
+@ApiStatus.Experimental
+public final class ActivityKeys {
+    /**
+     * {@code minecraft:core}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> CORE = create(key("core"));
+
+    /**
+     * {@code minecraft:idle}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> IDLE = create(key("idle"));
+
+    /**
+     * {@code minecraft:work}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> WORK = create(key("work"));
+
+    /**
+     * {@code minecraft:play}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> PLAY = create(key("play"));
+
+    /**
+     * {@code minecraft:rest}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> REST = create(key("rest"));
+
+    /**
+     * {@code minecraft:meet}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> MEET = create(key("meet"));
+
+    /**
+     * {@code minecraft:panic}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> PANIC = create(key("panic"));
+
+    /**
+     * {@code minecraft:raid}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> RAID = create(key("raid"));
+
+    /**
+     * {@code minecraft:pre_raid}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> PRE_RAID = create(key("pre_raid"));
+
+    /**
+     * {@code minecraft:hide}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> HIDE = create(key("hide"));
+
+    /**
+     * {@code minecraft:fight}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> FIGHT = create(key("fight"));
+
+    /**
+     * {@code minecraft:celebrate}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> CELEBRATE = create(key("celebrate"));
+
+    /**
+     * {@code minecraft:admire_item}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> ADMIRE_ITEM = create(key("admire_item"));
+
+    /**
+     * {@code minecraft:avoid}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> AVOID = create(key("avoid"));
+
+    /**
+     * {@code minecraft:ride}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> RIDE = create(key("ride"));
+
+    /**
+     * {@code minecraft:play_dead}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> PLAY_DEAD = create(key("play_dead"));
+
+    /**
+     * {@code minecraft:long_jump}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> LONG_JUMP = create(key("long_jump"));
+
+    /**
+     * {@code minecraft:ram}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> RAM = create(key("ram"));
+
+    /**
+     * {@code minecraft:tongue}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> TONGUE = create(key("tongue"));
+
+    /**
+     * {@code minecraft:swim}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> SWIM = create(key("swim"));
+
+    /**
+     * {@code minecraft:lay_spawn}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> LAY_SPAWN = create(key("lay_spawn"));
+
+    /**
+     * {@code minecraft:sniff}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> SNIFF = create(key("sniff"));
+
+    /**
+     * {@code minecraft:investigate}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> INVESTIGATE = create(key("investigate"));
+
+    /**
+     * {@code minecraft:roar}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> ROAR = create(key("roar"));
+
+    /**
+     * {@code minecraft:emerge}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> EMERGE = create(key("emerge"));
+
+    /**
+     * {@code minecraft:dig}
+     *
+     * @apiNote This field is version-dependant and may be removed in future Minecraft versions
+     */
+    public static final TypedKey<Activity> DIG = create(key("dig"));
+
+    private ActivityKeys() {
+    }
+
+    private static @NotNull TypedKey<Activity> create(final @NotNull Key key) {
+        return TypedKey.create(RegistryKey.ACTIVITY, key);
+    }
+}

paper-api-generator/src/main/java/io/papermc/generator/Generators.java

@@ -13,6 +13,7 @@
 import org.bukkit.generator.structure.StructureType;
 import org.bukkit.inventory.meta.trim.TrimMaterial;
 import org.bukkit.inventory.meta.trim.TrimPattern;
+import io.papermc.paper.entity.ai.Activity;
 
 public interface Generators {
 
@@ -23,6 +24,7 @@ public interface Generators {
         simpleKey("TrimPatternKeys", TrimPattern.class, Registries.TRIM_PATTERN, RegistryKey.TRIM_PATTERN, true),
         simpleKey("StructureKeys", Structure.class, Registries.STRUCTURE, RegistryKey.STRUCTURE, true),
         simpleKey("StructureTypeKeys", StructureType.class, Registries.STRUCTURE_TYPE, RegistryKey.STRUCTURE_TYPE, false),
+        simpleKey("ActivityKeys", Activity.class, Registries.ACTIVITY, RegistryKey.ACTIVITY, false),
         new MobGoalGenerator("VanillaGoal", "com.destroystokyo.paper.entity.ai")
     };