Add groups option to cluster proxy (#297)

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
-->
## 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 -->

pkg/applyconfigurations/api/v1alpha1/clusterproxyspec.go

@@ -15,6 +15,8 @@ type ClusterProxySpecApplyConfiguration struct {
 	APIServer *string `json:"apiServer,omitempty"`
 	// ServiceAccountName is a reference to the service account used for impersonation.
 	ServiceAccountName *string `json:"serviceAccountName,omitempty"`
+	// Groups are references to groups that the peer will be a part of.
+	Groups []GroupReferenceApplyConfiguration `json:"groups,omitempty"`
 }
 
 // ClusterProxySpecApplyConfiguration constructs a declarative configuration of the ClusterProxySpec type for use with
@@ -46,3 +48,16 @@ func (b *ClusterProxySpecApplyConfiguration) WithServiceAccountName(value string
 	b.ServiceAccountName = &value
 	return b
 }
+
+// WithGroups adds the given value to the Groups field in the declarative configuration
+// and returns the receiver, so that objects can be build by chaining "With" function invocations.
+// If called multiple times, values provided by each call will be appended to the Groups field.
+func (b *ClusterProxySpecApplyConfiguration) WithGroups(values ...*GroupReferenceApplyConfiguration) *ClusterProxySpecApplyConfiguration {
+	for i := range values {
+		if values[i] == nil {
+			panic("nil value passed to WithGroups")
+		}
+		b.Groups = append(b.Groups, *values[i])
+	}
+	return b
+}