Reintroduce dummy `.spec.clusterConfig.listenerClass`

[nightkr]

There was only one valid value, but some of our tests and examples try to set it explicitly. After some discussion we agreed to add it back as a dummy field for now.

Regressed in #450, reported at https://stackable-workspace.slack.com/archives/C031A56R127/p1706516386716159.

[lfrancke]

Can you elaborate what the future of this is?

Do we want to remove this field in the next version or will it stay around forever?

[nightkr]

It should be dropped in v1alpha2.

[lfrancke]

@sbernauer Could you add this to your tracking issue for future changes?