KEYCLOAK-3618: Extensible ResourceType for provider-created AdminEvents

[dteleguin]

The implementation is based on the extensible enum pattern. It requires that the enum implement an interface (an empty one in our case).

There were two options for refactoring:

1. leave ResourceType as is, introduce IResourceType:
   public enum ResourceType implements IResourceType
   This mostly requires changes to imports and method signatures (about 20 code locations), but introduces that ugly IResourceType. ResourceType enum fields are referenced just like before, with ResourceType.of("FOO") used to obtain custom value;

2. make ResourceType interface, rename ResourceType to something like StandardResourceType:
   public enum StandardResourceType implements ResourceType
   This would require changes to all the enum field references (~100 locations).

I've implemented the first variant because of its relative simplicity. Please let me know if the second variant is preferable, I'll reimplement the PR.

[stianst]

I'm going to close this. We can't have static methods exposed on random classes. This is not a good approach.