Introduce distribution installer finalize function

[ragazenta]

This adds an ability to do something "after" installing is done. The current use case is for Debian based distribution to install APT sources files if needed.

Closes: #3715

[behrmann]

The plain name finalize is not the best choice, because we have a lot of finalization functions, so it begs the question finalize what and makes it hard to grep for?

[DaanDeMeyer]

Closing in favor of https://github.com/systemd/mkosi/pull/3963