Add IExtensibleEnum#init, extend FillerBlockType/PlacementBehaviour

src/main/java/net/minecraftforge/common/IExtensibleEnum.java

@@ -41,4 +41,10 @@ package net.minecraftforge.common;
  */
 public interface IExtensibleEnum
 {
+    /**
+     * Called by generated factory code to do any post-constructor setup required by
+     * the enum. Should not be called manually.
+     */
+    @Deprecated
+    default void init() {}
 }