Allow restoring a specific backup with server-side backups

What does this MR do and why?

#422971 (closed)

Pass through the backup ID to gitaly-backup so that specific backups can be restored for server-side backups. The backup ID is still ignored when restoring non-server-side backups.

How to set up and validate locally

1. Enable server-side backups in gdk.yml:

   object_store:
     enabled: true

2. Rejigger gdk:

   $ gdk reconfigure && gdk restart

3. Make sure minio has a bucket called gitaly-backups. Visit http://127.0.0.1:9000 user: minio, pass: gdk-minio.
4. Create a full backup with REPOSITORIES_SERVER_SIDE=true. Skip db to be less invasive and faster.

   $ bundle exec rake gitlab:backup:create SKIP=db REPOSITORIES_SERVER_SIDE=true
   ...
   2023-10-02 22:48:41 UTC -- Backup 1696286904_2023_10_02_16.5.0-pre is done.
   2023-10-02 22:48:41 UTC -- Deleting backup and restore PID file ... done

5. Make some obvious changes to a repository. We'll use this to determine if the previous backup was restored or the backup we're about to create.
6. Create another full backup with REPOSITORIES_SERVER_SIDE=true. Note the different backup ID.

   $ bundle exec rake gitlab:backup:create SKIP=db REPOSITORIES_SERVER_SIDE=true
   ...
   2023-10-02 22:54:58 UTC -- Backup 1696287282_2023_10_02_16.5.0-pre is done.
   2023-10-02 22:54:58 UTC -- Deleting backup and restore PID file ... done

7. We now have two different backups. In this example the first 1696286904_2023_10_02_16.5.0-pre and the second 1696287282_2023_10_02_16.5.0-pre. We should be able to restore the first and the obvious changes go away, then restore the second and the obvious changes come back.

   $ bundle exec rake gitlab:backup:restore BACKUP=1696286904_2023_10_02_16.5.0-pre
   ...

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.