[new feature] GitHub Integration: PR comments for tests & sandbox states

[foxish]

New feature or functionality
What would you like Signadot to do?

Allow GitHub integration to optionally also post comments, summarizing Pull Request state with respect to sandboxes and smarttests.

[foxish]

This is now completed: https://www.signadot.com/docs/guides/integrate-ci/github#displaying-sandbox-status-in-github-prs