Update HCPEtcdBackup enhancement: rename CRD, add KMS-SSE and feature gate

- Rename EtcdBackup → HCPEtcdBackup throughout (resolve open question #2)
- Add kmsKeyARN (S3) and encryptionKeyURL (Azure) to HCPEtcdBackup spec
  for per-tenant artifact encryption (SSE-KMS / CMK), immutable fields
  populated by controller from HCP (resolve open question #4)
- Add HCPEtcdBackupConfig struct to ManagedEtcdSpec with KMS key fields,
  documenting HC → HCP → CR propagation chain
- Add Feature Gate section: HCPEtcdBackup gate with controller predicate,
  +openshift:enable:FeatureGate=HCPEtcdBackup annotations on all new fields
- Update graduation criteria with feature gate milestones

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Juan Manuel Parrilla Madrid <jparrill@redhat.com>

Author: jparrill