pybind/mgr/volumes: mirror (scheduled) snapshots to remote target
outline detailed here: https://pad.ceph.com/p/cephfs-snap-mirroring
mirror scheduled (and temporary) snapshots a remote target (identified by target-filesystem and target-path)
- schedule a snapshot to be mirrored based on a schedule (schedule could be different than the snapshot spec used to schedule snapshots).
$ ceph fs snap-mirror <source> <target> <schedule>