mgr/volumes: Add interface to get subvolume metadata

mgr/volumes: Add interface to get subvolume metadata

The following interface is added

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

The output is in json format with following fields

1. atime: access time of subvolume path in the format "YYYY-MM-DD HH:MM:SS"
2. mtime: modification time of subvolume path in the format "YYYY-MM-DD HH:MM:SS"
3. ctime: change time of subvolume path in the format "YYYY-MM-DD HH:MM:SS"
4. uid: uid of subvolume path
5. gid: gid of subvolume path
6. mode: mode of subvolume path
7. mon_addrs: list of monitor addresses
8. bytes_pcent: quota used in percentage if quota is set, else displays "undefined"
9. bytes_quota: quota size in bytes if quota is set, else displays "infinite"
10. bytes_used: current used size of the subvolume in bytes
11. created_at: time of creation of subvolume in the format "YYYY-MM-DD HH:MM:SS"
12. data_pool: data pool the subvolume belongs to
13. path: absolute path of a subvolume
14. type: subvolume type indicating whether it's clone or subvolume

Fixes: https://tracker.ceph.com/issues/44277
Signed-off-by: Kotresh HR <khiremat@redhat.com>