v1beta2: Class Namespace API change

[Danil-Grigorev]

We need to implement update to the API definitions of the Cluster object in order to properly leverage cross-namespace ClusterClass references.

Proposed design change:

topology:
  class:
    name: ...
    namespace: ...

which will deprecate introduced addition of the classNamespace field:

topology:
  class: …
  classNamespace: ...

Updated API can leverage +kubebuilder:selectablefield annotation, to add support for server-side indexing (available in k/k 1.32 by default but allowed in CRD schema from version 1.30).

/kind api-change

[chrischdi]

/triage accepted
/priority important-longterm