WIP: support repository_custom_properties resource and datasource

[felixlut]

Resolves #1956 (doesn't resolve that whole issue, but contributes to it)

Custom properties are defined on an organizational level, and then applied on a repository level. This PR aims to solve the latter of those two, while the former already has a PR for it in #2107.

This is a work in progress (WIP) since the underlying sdk:s are lacking for the REST endpoints pertaining to the repository level custom properties:

• Regular go-github lacks support for the multi_select custom property type, see https://github.com/google/go-github/issues/3198. There is a PR close to being merged for solving this https://github.com/google/go-github/pull/3200
  • The PR is merged, but no new release of go-github has been released since
• As @kfcampbell mentioned here, the GitHub SDK team has created a SDK generated from GitHub's OpenAPI spec. I've added and used it to implement a github_repository_custom_properties datasource with it due to the lacking functionality of go-github in this area currently. I'm currently trying to get it to write the custom properties, which I've been unsuccessful in so far. See https://github.com/octokit/go-sdk/issues/90
  • Reworked the github_repository_custom_properties datasource to github_repository_custom_property (notice propertIES --> propertY), which fetches a specific property instead of all of them after discussion in https://github.com/integrations/terraform-provider-github/pull/2107#discussion_r1677557505

If there's any progress on either of these I'll update this PR

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Before the change?

After the change?

Pull request checklist

• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• [ ] Yes
• [ ] No

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