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

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.

Start by mapping the current backend archiving flow, the Nextcloud integration, and the UI responsibilities described in the issue. Define the scope and boundaries for external archiver plugins, including how Borg-style encryption, deduplication, listing, pruning, and mounting would be represented; done means the plugin contract and affected integration points are agreed.