Adding a computed attribute for the repository owner to the github_repository

[AlanCitrix]

Resolves #2503

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Before the change?

• The repository owner has to either be parsed out of the full_name, or passed around as a separate variable.

After the change?

• Repository owner is now a computed field in both the repository resource and data source.
• The data source can be queried by name + owner when using anonymous auth. full_name will also still work.

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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