properly bind ClassSet extensions as singletons

The ExtensionPoint.ClassSet binds adds the extension classes to a a Multibinder and binds
the classes and calls the asEagerSingleton method on the multibinder. This does not actually
create a singleton. Instead we first bind the class as a singleton and add then add the class
to the multibinder.

Closes #14194

core/src/main/java/org/elasticsearch/common/util/ExtensionPoint.java

@@ -191,7 +191,8 @@ public abstract class ExtensionPoint {
         protected final void bindExtensions(Binder binder) {
             Multibinder<T> allocationMultibinder = Multibinder.newSetBinder(binder, extensionClass);
             for (Class<? extends T> clazz : extensions) {
-                allocationMultibinder.addBinding().to(clazz).asEagerSingleton();
+                binder.bind(clazz).asEagerSingleton();
+                allocationMultibinder.addBinding().to(clazz);
             }
         }
     }