added support for a repository proxy

[tosmi]

As part of your pull request, please review the following tasks:

• [X ] Signed the CLA (if not already signed)
• [ X] Rebased/up-to-date with base branch
• [ X] Tests pass

currently only for redhat yum repos

[tylerjl]

This looks great, @tosmi. Thank you for the tests as well 👍

You mentioned you signed the CLA, though the check doesn't seem to be passing - not sure whether that's on our side or not, I'm looking into it. Note that the CLA check will verify against the author information in the git commit.

[tosmi]

hm, ok the cla ask for my gitlab username, which is tosmi. the email address i'm using in my commit's is toni at stderr dot at. maybe that's a problem... drop me a note if i can do anything.