restic: on-demand backups

[eliasp]

Allow on-demand backups for subsets of a global backup.

The goal is, to allow subsets of a globally configured backup to be triggered on-demand, e.g. trigger a backup of ~/.password-store whenever an entry is modified.

Triggering the backup could happen for example using a .path unit monitoring the target.

The implementation would be based on a instance template service unit, which replicates the unit of the global backup, but instead of hardcoding target paths for the backup it leaves them to the instance specifier, so a path unit monitoring a given path could pass the backup target using Unit=restic-backups-foobar@escaped-target-path.service