This change adds an option to specify groups a cluster proxy peer is a
member of.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
* **New Features**
* Added optional `spec.groups` to the ClusterProxy custom resource to
associate group references by `id`, `name`, or `localRef`.
* CRD schema includes OpenAPI validation to enforce that each group
reference specifies exactly one selector.
* **Bug Fixes**
* Improved deep-copy behavior for ClusterProxy spec so group references
are copied safely and don’t share underlying slices.
* **Documentation**
* Updated API reference docs and README API table to include the new
`groups` field.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->