Go-github: Support self-hosted runner groups

[davidkarlsen]

Implement API for https://docs.github.com/en/rest/reference/actions#self-hosted-runner-groups

[nightlark]

I've started looking at implementing support for self-hosted runner groups, though this week is pretty busy so I probably won't get a PR open for it until end of next weekend.

[nightlark]

Sorry, I started adding support for this API then work got busy -- estimating that I'll have time to finish work on it towards the end of next week.

[davidkarlsen]

It's open-source - I fully understand this cannot be main concern at any given time! Thanks and good luck :)