Backup from Cloud Service

[rmanibus]

What does this PR change? What problem does it solve?

This is an attempt to implement #3434
It is a work in progress, I am posting to create a discussion and get feedback

Was the change previously discussed in an issue or on the forum?

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.

[MichaelEischer]

Just a quick comment: I didn't have time yet to take a closer look at the proposal in #3434. Based on a quick glance, I'm highly skeptical about the maintainability of this approach: it likely won't stay at local filesystem and gdrive as backup sources, which makes me worry about the scalability of this approach. We're already only barely able to keep up with PRs and issues.

Please don't invest further time into this PR until I've had time to take a closer look at the overall architecture.