[enh] Backup: refactor, and add borg method

[kay0u]

The problem

...

Solution

• Move some logic to BackupMethod subclasses
• Add BorgBackupMethod

PR Status

Kind of tested, with tar and borg (only with a localhost repo for now)
There is no unit tests yet
We should use BORG_FILES_CACHE_SUFFIX, as it's done in borg_ynh app

How to test

yunohost settings set misc.backup.default_backup_method -v borg
yunohost backup create --debug

# You can start some borg commands too with:
borg list /home/yunohost.backup/archives/borg::20251118-231311 # With the ID of the backup

yunohost backup list

yunohost backup info 20251118-231311

yunohost backup restore 20251118-231311 --apps

We can set a remote repo with something like that:

yunohost settings set misc.backup.borg.repository_path -v "user@backup-server:/backups/yunohost"
yunohost settings set misc.backup.borg.rsh -v "ssh -i /root/.ssh/backup_key"
yunohost settings set misc.backup.borg.passphrase -v "mon_super_mot_de_passe"