support cephfs snapdiffs

[jessegalley]

I see that the python bindings are up to date with the snapdiff feature added in Reef, but go-ceph doesn't have these functions (ceph_open_snapdiff(), ceph_readdir_snapdiff(), etc)

It would be great if we could get support for these snapdiff functions in go-ceph.

[phlogistonjohn]

Implementation of python bindings referenced:
https://github.com/ceph/ceph/blob/6273f1fc320181080f5873437ce0e464a77004d8/src/pybind/cephfs/cephfs.pyx#L342
https://github.com/ceph/ceph/blob/6273f1fc320181080f5873437ce0e464a77004d8/src/pybind/cephfs/cephfs.pyx#L1045