[Feature] Add support for external archiver-plugins (to support using borg-backup)

[neufeind]

With plugins taking over the actual "archiving" I think it might be possible to still use a lot from the UI and integration with Nextcloud, but provide for a solid backup-funktionality in the backend. Using borg-backup things like encryption, deduplication and compression are already handled by a performant and proven stable tools. Left to this plugin would be nextcloud-specific things like sqldumps, metadata, deciding which folders to backup or the UI-part. Borg offers functionality to access and manage backups, to list or prune backups, if need be even "fuse-mount" a particular backup. Externally all backups appear like "full backups" although they are stored efficiently, even encrypted, and handled transparently to the application.

[tomck]

Since the Nextcloud All-In-One offers Borg Backup built in, this seems very reasonable. https://github.com/nextcloud/all-in-one