mgr/volumes: Add snapshot info command

mgr/volumes: Add snapshot info command

The following command is added

"ceph fs subvolume snapshot info <vol_name> <sub_name> <snap_name> [<group_name>]"

The output is in json format with following fields

    created_at: time of creation of snapshot in the format "YYYY-MM-DD HH:MM:SS:ffffff"
    data_pool: data pool the snapshot belongs to
    has_pending_clones: "yes" if snapshot clone is in progress otherwise "no"
    protected: "yes" if snapshot is protected otherwise "no"
    size: snapshot size in bytes

Fixes: https://tracker.ceph.com/issues/45237
Signed-off-by: Kotresh HR <khiremat@redhat.com>