mgr/ceph_secrets: add SecretMgr for secrets handling and resolution

mgr/ceph_secrets: add SecretMgr for secrets handling and resolution

Introduce SecretMgr to encapsulate higher-level behavior on top of
SecretStoreMon: listing helpers, scan_refs, scan_unresolved_refs, and
resolve_object (walk nested dict/list structures). This keeps
parsing/substitution logic out of the mgr module entrypoint and makes
consumer behavior consistent. The module can now resolve secret://…
references deterministically and provide structured scan output.

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>