Add support for proxy with githubapp authentication method

[samirtahir91]

Checklist

• I did not find a related open enhancement request.
• I understand that enhancement requests filed in the GitHub repository are by default low priority.
• If this request is time-sensitive, I have submitted a corresponding issue with GCP support.

Describe the feature

Hi

I am seeing validation fail with Nomos, for a RootSync with spec.git.auth as githubapp and spec.git.proxy set.

I have raised PR - #1796

Please can you review/suggest any other changes required for this.

Thanks

Samir

Importance

This blocks use of proxy for GitHub sources when authenticating with a GitHub app