api: Target schema should be case insensitive

URI schema are case-insensitive. Previously the code would do
case-sensitive matching. We expect NameResolverProviders to return the
typical canonical scheme formatting, which is lower-case. If a
NameResolverProvider returns an unexpected string (upper case, unicode,
etc), then it simply won't ever match. Channel users, however, can use
either casing in target strings.

The code implicitly already handled relative URIs by returning null, as
Map.get(null) returned null.

api/src/main/java/io/grpc/NameResolverProvider.java

@@ -61,6 +61,8 @@ public abstract class NameResolverProvider extends NameResolver.Factory {
    * delegates to {@link Factory#getDefaultScheme()} before {@link NameResolver.Factory} is
    * deprecated in https://github.com/grpc/grpc-java/issues/7133.
    *
+   * <p>The scheme should be lower-case.
+   *
    * @since 1.40.0
    * */
   protected String getScheme() {

api/src/main/java/io/grpc/NameResolverRegistry.java

@@ -26,6 +26,7 @@
 import java.util.HashMap;
 import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.logging.Level;
 import java.util.logging.Logger;
@@ -162,7 +163,11 @@ private final class NameResolverFactory extends NameResolver.Factory {
     @Override
     @Nullable
     public NameResolver newNameResolver(URI targetUri, NameResolver.Args args) {
-      NameResolverProvider provider = providers().get(targetUri.getScheme());
+      String scheme = targetUri.getScheme();
+      if (scheme == null) {
+        return null;
+      }
+      NameResolverProvider provider = providers().get(scheme.toLowerCase(Locale.US));
       return provider == null ? null : provider.newNameResolver(targetUri, args);
     }
 

api/src/test/java/io/grpc/NameResolverRegistryTest.java

@@ -183,8 +183,12 @@ public NameResolver newNameResolver(URI passedUri, NameResolver.Args passedArgs)
         });
 
     assertThat(registry.asFactory().newNameResolver(uri, args)).isNull();
+    assertThat(registry.asFactory().newNameResolver(URI.create("/0.0.0.0:80"), args)).isNull();
+    assertThat(registry.asFactory().newNameResolver(URI.create("///0.0.0.0:80"), args)).isNull();
     assertThat(registry.asFactory().newNameResolver(URI.create("other:///0.0.0.0:80"), args))
             .isSameInstanceAs(nr);
+    assertThat(registry.asFactory().newNameResolver(URI.create("OTHER:///0.0.0.0:80"), args))
+            .isSameInstanceAs(nr);
     assertThat(registry.asFactory().getDefaultScheme()).isEqualTo("dns");
   }