feat: support github enterprise api

[ricardojdsilva87]

Pull Request

Proposed Changes

This Pull request is based on the already merged https://github.com/github/evergreen/pull/256 to give support to GitHub Enterprise login using a GitHub App also created on GitHub Enterprise

• Split the authentication to a separate file
• Added needed tests

Code coverage

---------- coverage: platform darwin, python 3.13.0-final-0 ----------
Name             Stmts   Miss  Cover   Missing
----------------------------------------------
auth.py             16      1    94%   47
stale_repos.py     151     16    89%   142-143, 184-185, 200-201, 224, 326, 328, 335-336, 394-395, 402-403, 413
----------------------------------------------
TOTAL              167     17    90%

Required test coverage of 80% reached. Total coverage: 89.82%

Readiness Checklist

Author/Contributor

• [x] If documentation is needed for this change, has that been included in this pull request
• [x] run make lint and fix any issues that you have introduced
• [x] run make test and ensure you have test coverage for the lines you are introducing
• [ ] If publishing new data to the public (scorecards, security scan results, code quality results, live dashboards, etc.), please request review from @jeffrey-luszcz

Reviewer

• [ ] Label as either fix, documentation, enhancement, infrastructure, maintenance or breaking