Support for signed repos

[Frzk]

The buildpack currently allows to add a repo, but there's no easy way to add a signed repo.
Since these are more and more widespread, it would be nice to support them too.

[EtienneM]

There is currently a workaround for this. One can add trusted=yes when adding a repository in order to automatically trust he repository. For example:

:repo:deb [trusted=yes] https://cli.github.com/packages stable main

This still leads to a warning in the deployment logs:

W: GPG error: https://cli.github.com/packages stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 23F3D4EA75716059

But the repository is still usable.

[EtienneM]

In case we actually want to support the public key, here is a comment from someone who implemented it in a fork: heroku#98 (comment)

[Frzk]

🤯 🤯 🤯
I've never heard about such an option.
We should really document it (at least).