testing: add tiny framework for testing rustic's compat with latest restic

[simonsan]

This is the first PR to lay some foundations to test rustic's compatibility against a restic repository. It should essentially show, how to start out with it and give some fixtures to play around with.

TODO:

• [X] add CI workflow + test that uses restic (latest) via https://github.com/AnimMouse/setup-restic, to create a new repo and run rustic against it
• [X] investigate AnimMouse/setup-restic failure and fix
  • [X] forked AnimMouse/setup-restic to rustic-rs/setup-restic to run on @main and apply faster fixes to our CI
  • [X] used AnimMouse/setup-restic as a foundation for rustic-rs/setup-rustic, still WIP
• [x] update to 'release' feature in test when #1307 is merged