Split out the repo syncing logic into standalone binary

For bumpbuddys out of date flagging logic we also want a synced copy of all repositories 😊 To properly implement this and keep it DRY it would be nice to have one implementation for multiple usecases and IMO a good solution would be to export the buildbtw tooling for this into it's own binary that could then be invoked via systemd-timers.

I'd be open to help with this (also with the implementation) once the buildbtw project is in a suitable/stable enough place that this binary could be derived from it.