mgr/volumes: set, get, list and remove custom metadata for snapshot

mgr/volumes: set, get, list and remove custom metadata for snapshot

If CephFS in ODF configured in external mode, user like to use
subvolume snapshot metadata to store some Openshift specific
information, as the PVC/PV/namespace the subvolumes/snapshot
are coming from. For RBD volumes, it's possible to add metadata
information to the images using the 'rbd image-meta' command.
However, this feature is not available for CephFS volumes.
We'd like to request this capability.

Adding following commands:
    ceph fs subvolume snapshot metadata set <vol_name> <sub_name> <snap_name> <key_name> <value> [<group_name>]
    ceph fs subvolume snapshot metadata get <vol_name> <sub_name> <snap_name> <key_name> [<group_name>]
    ceph fs subvolume snapshot metadata ls <vol_name> <sub_name> <snap_name> [<group_name>]
    ceph fs subvolume snapshot metadata rm <vol_name> <sub_name> <snap_name> <key_name> [<group_name>] [--force]

Fixes: https://tracker.ceph.com/issues/55401
Signed-off-by: Nikhilkumar Shelke <nshelke@redhat.com>