feat: Report as issue markdown

[zkoppert]

Pull Request

fixes #11

Proposed Changes

This pull request introduces a new feature that generates a report of users removed from the CODEOWNERS files across the repos that were affected. The report is written in Markdown format and can be optionally created as an issue in the repository. The ISSUE_REPORT environment variable controls this feature. The README.md file has been updated to explain this new feature and how to use it.

I also added/modified tests to cover the new functionality.

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
• [x] If publishing new data to the public (scorecards, security scan results, code quality results, live dashboards, etc.), please request review from @jeffrey-luszcz

Reviewer

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

[jmeridth]

Ran lint locally and got cleanowners.py:145:58: E0601: Using variable 'codeowners_file_contents_new' before assignment (used-before-assignment) hence the change to default codeowners_file_contents_new to None in my commit.

[zkoppert]

Looks good! Thanks for adding @jmeridth. Leaving this pr open for @sutterj to take a look at.