[Feature request] Custom App Backup solutions implementation

[gromoslaw-kroczka]

Is your feature request related to a problem? Please describe.
Currently script backups only docker volumes and bind mounts. Many Apps include own backup systems, for example:

• Paperless-ngx
• Nextcloud
• Home Assistant
  etc.

Those backups should be lighter in size and easier to use for example to restore in different environment.

Describe the solution you'd like
Add option for custom backups of popular applications (as described above)

Describe alternatives you've considered
This is just additional option, to be implemented

Additional context
I've recently moved to Proxmox and start using whole VM snapshots as main backup. In this scenario per-app-backups will be supplementary ones.

Additional challange: implement that using Python scripting?