Add support for issue types

[stevehipwell]

The GitHub REST API now supports managing issue types.

• REST API endpoints for issue types

[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!

[atilsensalduz]

Hey guys, I'd like to work on this issue. Could you please assign it to me?

[gmlewis]

Thank you, @atilsensalduz! It's yours.

[atilsensalduz]

Hey guys, I've completed my work, but I stumbled upon a GHES API version upgrade. When I ran generate.sh, it updated the GHES version from 3.15 to 3.16. Should I include this change in my PR as well?

[gmlewis]

That should be fine, thank you for checking!