Add automated backup workflow

[SaschaMann]

This PR adds an automated weekly backup for all repo data, including issues and PRs. I'm adding these across all repos in julia-actions since I personally find it useful to not fully rely on GitHub for backups for my own projects, also for peace of mind. If you don't want this, just close the PR, I don't want to force these on anyone, just offer it :)

Details:

• The data is downloaded from the API using python-github-backup and only includes publicly accessible info,
• it is then passed into restic for deduplication and encryption,
• and finally it's uploaded into a Backblaze B2 Bucket (comparable to AWS S3 but cheaper).
• The credentials are accessible as org secrets. I'll happily create you an extra set of keys if you want, though.
• If you need to restore anything, you can either use the org secrets and restore it yourself, or ask me and I'll do it for you.
• The B2 bucket is on very strict cost limits, so please don't increase the frequency of the builds.
• This is somewhat experimental. If it turns out to be too expensive, I may remove them again.