rbd/admin: add support for `ceph rbd migration` api

[Rakshith-R]

the ceph cli tools supports commands like:

ceph rbd migration prepare/execute/commit/abort

These can be done via the "json command" api for the mgr. We can support that functionality in rbd/admin.

Refer https://docs.ceph.com/en/latest/rbd/rbd-live-migration/?highlight=migration

Adding above apis should also enable us to add corresponding task apis

• rbd task migration [abort|commit|execute] continuing from #516