mgr/ceph_secrets: add storage backend protocol for mgr KV secrets

mgr/ceph_secrets: add storage backend protocol for mgr KV secrets

Define a minimal backend protocol for secret persistence
operations (get/set/rm/list), keeping the module implementation
decoupled from the backing store details. For now we will start with
monstore-db as secure KV store but the idea is to extend this to other
backends such as Vault.

Fixes: https://tracker.ceph.com/issues/74562
Assisted-by: Claude <claude.ai>
Assisted-by: ChatGPT <chatgpt.com>
Signed-off-by: Redouane Kachach <rkachach@ibm.com>