Add a new data movement layer to Velero's architecture

[nrb]

Describe the problem/challenge you have

Velero has restic integration as part of it's core, which is useful for moving filesystem level data between storage providers or when a Velero volume snapshot plugin isn't available for a storage platform. However, since the restic integration is part of Velero's core, this is also challenging should someone like to use an alternative solution such as Borg, Duplicity, or Kopia.

Describe the solution you'd like

Velero should provide a more abstract, swappable layer for allowing filesystem data transfer out of persistent volume. This should abstract the following things:

• the tool used to send the data (for example, replace restic with some other executable)
• the destination for filesystem data (instead of being forced to use restic repositories, allow using something like Borg scp endpoints)

Anything else you would like to add:

An implementation of a data mover already exists in the vSphere plugin. It can be used as a reference for bringing this into Velero.

See issues #3223 and #3226 for requests to add replacement tools to Velero.

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[reasonerjt]

We'll do some investigation in the timeframe of v1.9 to better understand the gap but the actual scope may be different from the OP.

[reasonerjt]

We are not going to deliver the feature but only have the concrete design in v1.10, and that's tracked by another issue.