feat(backup): Add restic backup

Restic backup creates a snapper snapshot of the root system which it
then chroots into and starts a restic backup to a (wasabi) S3 bucket to.

Intended to roughly follow this
<https://codeberg.org/silmaril/my-restic-solution> solution to achieve
restic backup of the _newest_ snapshot of my live root system.

roles/backup/defaults/main.yaml

@@ -1,2 +1,12 @@
 ---
 # user_name: # required for snapper templates
+
+restic_repository:
+restic_password:
+restic_s3_id:
+restic_s3_key:
+
+restic_daily: 3
+restic_weekly: 2
+restic_monthly: 6
+restic_yearly: 1