Manage apt_auth.conf from cloud init

It is possible to manage the package sources from cloud-init, but I haven't encountered any documentation concerning the setup of the [apt_auth.conf](https://manpages.debian.org/testing/apt/apt_auth.conf.5.en.html) file.

If it is not handled yet, is this the proper channel to make a feature request? I could also participate to the redaction of a pull request if it can help.