Use CEL validation in our CRDs

[scholzj]

We should consider using CEL validation in our CRDs to improve the validation of things that OpenAPI cannot validate. Two things were raised recently:

• Validation of unique listener names (#9338)
• Validation of fields required only for some type that cannot be set as required in OPenAPI because of other type values (#9398)

We should investigate how CEL can help there. But we should keep in mind the following aspects:

• It would be good to understand first how you plan to generate the validation rules from the Java code (as you cannot simply just edit the CRD and add it there -> they are all generated from the code)
• We have to make sure it scales well when used for all fields like this which there are quite a many
• We have to make sure it does not break the CRDs on the older Kubernetes versions. I'm not sure if this breaks support of older versions or not - but some new features in the past did it. So we need to be careful about it.

[scholzj]

Discussed in the community call on 14.12.: Could provide a useful functionality. Might need a proposal depending on the complexity of the added rules and how are the generated out of the API classes?