Merge PR #35647 into master

* refs/pull/35647/head:
	mgr/volumes: Add documentation regarding --retain-snapshots option
	mgr/volumes: Avoid trashing retained subvolume on create errors
	mgr/volumes: Add subvolume v2 test cases
	mgr/volumes: Derive v2 from v1 to leverage common methods
	mgr/volumes: Introduce v2 subvolumes
	mgr/volumes: Use operation type during subvolume open

Reviewed-by: Ramana Raja <rraja@redhat.com>
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Reviewed-by: Patrick Donnelly <pdonnell@redhat.com>
Reviewed-by: Kotresh Hiremath Ravishankar <khiremat@redhat.com>

diff --cc qa/tasks/cephfs/test_volumes.py
Simple merge

diff --cc src/pybind/mgr/volumes/fs/volume.py
Simple merge