mgr/volumes: get the list of auth IDs that have been granted access to a subvolume using mgr/volumes CLI
Manila requires the list of auth IDs that have been granted access to a share/subvolume.
The ceph_volume_client's get_authorized_ids() provides this functionality. https://github.com/ceph/ceph/blob/octopus/src/pybind/ceph_volume_client.py#L1308
The ceph_volume_client stores the auth IDs that have been given access to a subvolume in special metadata files, and fetches the list of auth IDs when requested. mgr/volumes needs to provide the same functionality. It needs to store and fetch auth IDs in the same special metadata files used by the ceph_volume_client. This will allow the manila driver to smoothly switch to using mgr/volumes from ceph_volume_client.