feat(schema): guided and AutoYaST storage proposals (#1263)

Schema definition for the guided and AutoYaST storage proposals:

~~~json
{
  "storage": {
    "guided": { }
  },
  "legacyAutoyastStorage": [ ]
}
~~~

Based on
[auto_storage.md](https://github.com/openSUSE/agama/blob/master/doc/auto_storage.md)
which is expected to change over time.

Bonus: This includes a commit revisiting how `title` and `description`
are used, to be more compliant with
https://json-schema.org/understanding-json-schema/reference/annotations

Note for reviewers: you can easily check your JSON config files by using
`agama profile validate config.json`

rust/agama-lib/share/examples/profile_tw.json

@@ -12,7 +12,12 @@
     "id": "Tumbleweed"
   },
   "storage": {
-    "bootDevice": "/dev/dm-1"
+    "guided": {
+      "boot": {
+        "configure": true,
+        "device": "/dev/dm-1"
+      }
+    }
   },
   "user": {
     "fullName": "Jane Doe",

rust/agama-lib/share/examples/storage.json

@@ -0,0 +1,63 @@
+{
+  "storage": {
+    "guided": {
+      "target": {
+        "disk": "/dev/vdc"
+      },
+      "boot": {
+        "configure": true,
+        "device": "/dev/vda"
+      },
+      "encryption": {
+        "password": "notsecret",
+        "method": "luks2",
+        "pbkdFunction": "argon2i"
+      },
+      "space": {
+        "policy": "custom",
+        "actions": [
+          { "resize": "/dev/vda" },
+          { "forceDelete": "/dev/vdb1" }
+        ]
+      },
+      "volumes": [
+        {
+          "mount": {
+            "path": "/",
+            "options": ["ro"]
+          },
+          "filesystem": {
+            "btrfs": {
+              "snapshots": true
+            }
+          },
+          "size": [1024, "5 Gib"],
+          "target": "default"
+        },
+        {
+          "mount": {
+            "path": "/home"
+          },
+          "filesystem": "xfs",
+          "size": {
+            "min": "5 GiB",
+            "max": "20 GiB"
+          },
+          "target": {
+            "newVg": "/dev/vda"
+          }
+        },
+        {
+          "mount": {
+            "path": "swap"
+          },
+          "filesystem": "swap",
+          "size": "8 GiB",
+          "target": {
+            "newPartition": "/dev/vda"
+          }
+        }
+      ]
+    }
+  }
+}