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

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

If CephFS in ODF configured in external mode, user like to use
volume / subvolume metadata to store some Openshift specific
information, as the PVC / PV / namespace the volumes / subvolumes
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 metadata set <vol_name> <sub_name> <key_name> <value> [<group_name>]
    ceph fs subvolume metadata get <vol_name> <sub_name> <key_name> [<group_name>]
    ceph fs subvolume metadata ls <vol_name> <sub_name> [<group_name>]
    ceph fs subvolume metadata rm <vol_name> <sub_name> <key_name> [<group_name>] [--force]

Fixes: https://tracker.ceph.com/issues/54472
Signed-off-by: Nikhilkumar Shelke <nshelke@redhat.com>
(cherry picked from commit fb8865194a0f3a74b1ca9ade9be7f401eb55e7d6)