Add support for code coverage rules

[stevehipwell]

GitHub has announced REST API support for managing code coverage with rulesets.

[gmlewis]

Thank you, @stevehipwell!

This would be a great PR for any new contributor to this repo or a new Go developer.
All contributions are greatly appreciated!

Feel free to volunteer for any issue and the issue can be assigned to you so that others don't attempt to duplicate the work.
Also feel free to perform a code review on any PR that has the label "NeedsReview" - we appreciate those contributions too!
We have a REVIEWERS file that you could add your username to if you are willing to be contacted when reviews are needed.

Please check out our CONTRIBUTING.md guide to get started. (In particular, please remember to go generate ./... and don't use force-push to your PRs.)

Thank you!

[gmlewis]

@Bobrovskii - please comment on this issue so that I can assign it to you as explained in CONTRIBUTING.md.

[Bobrovskii]

Sorry, I missed that step. I would like to work on this issue, and I have already opened PR #4603

[gmlewis]

Sorry, I missed that step. I would like to work on this issue, and I have already opened PR #4603

Thank you, @Bobrovskii! It's yours.