docs: set, get, list and remove custom metadata for snapshot

docs: set, get, list and remove custom metadata for snapshot

Set custom metadata on the snapshot as a key-value pair using
    $ ceph fs subvolume snapshot metadata set <vol_name> <subvol_name> <snap_name> <key_name> <value> [--group_name <subvol_group_name>]
    note: If the key_name already exists then the old value will get replaced by the new value.
    note: The key_name and value should be a string of ASCII characters (as specified in python's string.printable). The key_name is case-insensitive and always stored in lower case.
    note: Custom metadata on a snapshots is not preserved when snapshotting the subvolume, and hence, is also not preserved when cloning the subvolume snapshot.

Get custom metadata set on the snapshot using the metadata key::
    $ ceph fs subvolume snapshot metadata get <vol_name> <subvol_name> <snap_name> <key_name> [--group_name <subvol_group_name>]

List custom metadata (key-value pairs) set on the snapshot using::
    $ ceph fs subvolume snapshot metadata ls <vol_name> <subvol_name> <snap_name> [--group_name <subvol_group_name>]

Remove custom metadata set on the snapshot using the metadata key::
    $ ceph fs subvolume snapshot metadata rm <vol_name> <subvol_name> <snap_name> <key_name> [--group_name <subvol_group_name>] [--force]
    Using the '--force' flag allows the command to succeed that would otherwise fail if the metadata key did not exist.

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