provider/github: Add merge types to repository resource

[taiidani]

Adding merge types to the GitHub Repository resource so that users can control when the repository allows Squash merges, Rebase merges and/or Merge commits.

Fixes #15183

[taiidani]

Little birds everywhere are telling me that I'm going to need to move this to https://github.com/terraform-providers/terraform-provider-github >.<

[taiidani]

The contents of this PR have moved to https://github.com/terraform-providers/terraform-provider-github/pull/1 matching what appears to be the new location for the code. Should this PR be closed?

[apparentlymart]

Thanks for moving this over to the github provider repository, @taiidani! Indeed I'm going to close this now, since very soon this PR will no longer apply after the github code is removed from this repository.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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