Support for Repository Custom Properties

[liaodaniel]

GitHub has recently released https://github.blog/changelog/2023-10-12-github-repository-custom-properties-beta/. It would be nice to have support for this feature. With the feature in beta, we may expect API changes over time but I'll be happy to get a draft PR for now.

References:

• Properties API docs
• Repository Custom Properties Beta Feedback

[gmlewis]

Thank you, @liaodaniel !
It's yours.