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/tasks/main.yaml

@@ -2,4 +2,10 @@
   import_tasks: snapper.yaml
   tags:
     - btrfs
-    - snapshots
+    - snapper
+
+- name: Set up restic backups
+  import_tasks: restic.yaml
+  tags:
+    - btrfs
+    - restic