Add @listener annotation to provide better control of configuration in tests

[SamBarker]

This breaks the abstraction. The 'external' name is currently a private concern. We shouldn't need it.

Long term, I'd like the ext to support @Listener annotations that define the listeners that the Cluster exposes. It could permit arguments that would allow additional configuration to be supplied to the listener.

I think this is good enough for now.

Originally posted by @k-wall in kroxylicious/kroxylicious#1355 (comment)