add delete branch on merge option on datasource github repository

[alileza]

After the change?

Supporting a couple more fields from repository api

• delete_on_branch_merge
• get_allow_update_branch

Pull request checklist

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] 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
• [x] No

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[kfcampbell]

@alileza feel free to promote this from draft when you're comfortable!

[kfcampbell]

Would you mind adding checks for these new attributes to github/data_source_github_repository_test.go?