mgr/volumes: Derive v2 from v1 to leverage common methods

mgr/volumes: Derive v2 from v1 to leverage common methods

With v2 introduction in subvolumes, there is quite a bit of
common code and methods that both v1 and v2 share.

To reduce overall code bloat and improve maintainability, derive
SubvolumeV2 from SubvolumeV1.

Signed-off-by: Shyamsundar Ranganathan <srangana@redhat.com>
Conflicts:
	src/pybind/mgr/volumes/fs/operations/versions/subvolume_base.py
subvol pin operation not backported, required resolution
	src/pybind/mgr/volumes/fs/operations/versions/subvolume_v1.py
Updated features with Enum values