Add rdiff-backup replication method

[kyrofa]

Describe the feature you'd like to have.
The ability to replicate via rdiff-backup. This is our current backup method, but as we move more products to k8s, it has gotten harder to pull off. We started investigating writing an operator to take a snapshot and use rdiff-backup (or perhaps use SnapScheduler in combination with a more simple cron job), but then we discovered VolSync, which is almost a perfect fit except for this lack of support. Instead of building our own, I'd be happy to contribute this support to VolSync, but I wanted to first log an issue and gauge interest.

What is the value to the end user? (why is it a priority?)
rdiff-backup is a reverse incremental backup tool that uses the rsync protocol. It's not really suitable for replication, to be clear, but a way to take space-efficient backups. It generally works over SSH.

How will we know we have a good solution? (acceptance criteria)
A new replication method that looks a bit like the current rsync SSH method, something like the following:

---
apiVersion: volsync.backube/v1alpha1
kind: ReplicationSource
# snip...
spec:
  # snip...
  rdiff-backup:
    sshKeys: volsync-rdiff-dest-sshkeys
    address: my.host.com
    copyMethod: Clone

[tesshuflower]

@kyrofa I've discussed with @JohnStrunk and think we would potentially be interested in this if you're willing to contribute and help maintain the mover. Copying some comments John had from a request for a kopia mover:

By the end of the process, we need to have:

• Mover code (controller + mover parts)
• Documentation
• Tests

Since this will be a fairly substantial piece, starting w/ a design PR is probably a good place to begin.

We'd have a few questions at the start:

• Do you envision this as a backup/restore operation similar to the way restic works?
  • I.e. a replicationsource that connects via ssh to an existing rdiff server somewhere to backup
  • then a replicationdestination that would restore from that same server?
• Or is this going to be more of a point-to-point connection like rsync/rsync-tls where you have a source that connects to a destination started by volsync? In this case, do you ever plan on using rdiff restore functionality or would it just be assumed your destination pvc contains the latest files + the extra metadata from rdiff?
  • Additionally if you're looking into this route, I'd recommend you look at rsync-tls as a reference, as the old rsync ssh method will most likely get deprecated soon.

[Neustradamus]

To follow this ticket