Make the ControllerConfiguration annotation optional

[metacosm]

With workflows being moved to their own annotation, it might often happen that the ControllerConfiguration ends up being empty. It would be nice if it could be made optional.

Originally posted by @csviri in #2274 (comment)

[metacosm]

Note that, if I recall properly, we already looked at making it optional at some point but ran into some issues (though I don't remember which ones).
Note also that the annotation is already optional when you use the Quarkus extension because the processing is based on whether or not a class implements the Reconciler interface there.