Initial support for gitaly server-side backups

What does this merge request do and why?

Adds initial configuration to enable gitaly server-side backups using the included minio.

How to set up and validate locally

1. Switch GDK to this branch:

   $ cd <gdk_root>
   $ git switch gitaly_server_side_backups

2. Turn on object storage. gdk.yml:

   object_store:
     enabled: true

3. Reconfigure and restart.

   $ gdk reconfigure && gdk restart

4. Open minio in a browser and create a bucket named gitaly-backups if one doesn't already exist.

   • URL: http://127.0.0.1:9000
   • User: minio
   • Pass: gdk-minio

5. In gitlab, create a server-side backup (skip db so it's faster and less invasive):

   $ cd <gdk_root>/gitlab
   $ bundle exec rake gitlab:backup:create REPOSITORIES_SERVER_SIDE=true SKIP=db
   ...
   2023-08-09 03:02:04 UTC -- Backup 1691550107_2023_08_09_16.3.0-pre is done.
   2023-08-09 03:02:04 UTC -- Deleting backup and restore PID file ... done

6. You can browse minio and verify that a bunch of stuff was added 🎉 🎉

7. Try restore from said backup:

   $ bundle exec rake gitlab:backup:restore BACKUP=1691550107_2023_08_09_16.3.0-pre
   ...

8. The output will have a lot of "skipped" errors. This is just how repositories that don't exist are handled at the moment. Hopefully there will be some successful ones, they'll print completed restore:

   {"command":"restore","gl_project_path":"jashkenas/Underscore","level":"info","msg":"completed restore","relative_path":"@hashed/e7/f6/e7f6c011776e8db7cd330b54174fd76f7d0216b612387a5ffcfb81e6f0919683.git","storage_name":"default","time":"2023-08-09T03:06:48.899Z"}
   

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.