Draft: introduce restic for resiliency

This merge request supercedes !1794 (closed) that supports both rdiff-backup and restic.

You can configure backup software per PBS/clone with the following parameter.

    "pbs1-backup-software": {
      "title": "Backup software used for first clone and PBS.",
      "description": "Backup software used for first clone and PBS.",
      "type": "string",
      "enum": ["rdiff-backup", "restic"],
      "default": "rdiff-backup"
    },
    "pbs2-backup-software": {
      "title": "Backup software used for second clone and PBS.",
      "description": "Backup software used for second clone and PBS.",
      "type": "string",
      "enum": ["rdiff-backup", "restic"],
      "default": "rdiff-backup"
    },

• refactor stack/resilient so that we can easily extend it for other software than restic.
• support different backup software per pbs/clone.
• main instance should support pull backup from both rdiff-backup and restic.