xds: Provide default XdsResourceType.extractResourceName impl

This method is only needed sometimes, and with time will be needed less
and less. Don't require new types to implement it, instead relying on
control planes to use the new approach.

xds/src/main/java/io/grpc/xds/client/XdsResourceType.java

@@ -65,8 +65,18 @@ public abstract class XdsResourceType<T extends ResourceUpdate> {
   protected static final String TYPE_URL_TYPED_STRUCT =
       "type.googleapis.com/xds.type.v3.TypedStruct";
 
+  /**
+   * Extract the resource name from an older resource type that included the name within the
+   * resource contents itself. The newer approach has resources wrapped with {@code
+   * envoy.service.discovery.v3.Resource} which then provides the name. This method is only called
+   * for the old approach.
+   *
+   * @return the resource's name, or {@code null} if name is not stored within the resource contents
+   */
   @Nullable
-  protected abstract String extractResourceName(Message unpackedResource);
+  protected String extractResourceName(Message unpackedResource) {
+    return null;
+  }
 
   protected abstract Class<? extends com.google.protobuf.Message> unpackedClassName();