mgr/volumes: add `Dict` type

mgr/volumes: add `Dict` type

fixes mypy errors:

volumes/fs/operations/op_sm.py:39: error: "object" has no attribute "get"
volumes/fs/operations/op_sm.py:49: error: "object" has no attribute "get"

volumes/fs/operations/lock.py: note: In member "__init__" of class "GlobalLock":
volumes/fs/operations/lock.py:27: error: "object" has no attribute "__enter__"
volumes/fs/operations/lock.py:27: error: "object" has no attribute "__exit__"

volumes/fs/operations/lock.py: note: In member "lock_op" of class "GlobalLock":
volumes/fs/operations/lock.py:35: error: "object" has no attribute "__enter__"
volumes/fs/operations/lock.py:35: error: "object" has no attribute "__exit__"

Fixes: https://tracker.ceph.com/issues/44393
Signed-off-by: Michael Fritch <mfritch@suse.com>