Add ability to only transfer new snapshots during instance/volume refresh

[stgraber]

Currently using incus copy --refresh or incus storage volume copy --refresh will have Incus compare the snapshot list on the source and destination and transfer any snapshots that the destination is currently missing.

But sometimes the destination server is purposefully dealing with less snapshots as some have been manually removed by the user. To handle that, the API should be extended so the user can specify that only the new snapshots should be copied.

In this instance, new should be treated as any snapshot that was taken after the most recent existing snapshot on the target.

This should be possible with a new migration field, matching API field and CLI argument and by the migration layer looking at the snapshot creation time during negotiation.

[ps-gill]

If available, I would like to work on this issue.

[stgraber]

Sure, got it assigned to you!