Allow multiple borg repos per snapper config

[enzingerm]

There are use-cases where one would want to backup the same snapper config to multiple borg repositories, possibly with different encryption/retention settings. This is not possible at the moment, as snapborg_backup=true is written to the snapper snapshot userdata as soon as it has been backed up and this is obviously too little information if we have multiple borg repos.

This could be solved in 3 ways:

• Don't use snapper snapshot userdata at all and create a new database specific to snapborg where the mappings are stored.
• Encode the backup information in the snapper userdata
• Write the snapper snapshot ID in the borg backup metadata (name) and on each execution of snapborg run borg list beforehand to determine which snapshots have already been backed up

[jrabinow]

I think that adding backup info to the snapper userdata is probably not the way to go here, as there's many edge cases to handle. Off the top of my head:

• backup snapshots to multiple repos
• delete a backup from the borg archive and rerun snapborg

There's probably a few more, but in any case, adding metadata to snapper doesn't seem to me like it'll be a clean approach overall. If the purpose is just to determine which snapshots are already backed up, I would vote for using borg as source of truth. If there's any chance that this metadata might be extended in the future to include other info, I would vote for a new, separate db.

Just my $.02