Add method to get a single runner group

[MatisseHack]

Resolves #2705

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Behavior

Before the change?

• Octokit didn't have a method to get a single self-hosted runner group

After the change?

• That API has been implemented

Other information

• I also added a missing property to the model

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Additional info

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)
• Added the appropriate label for the given change

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes (Please add the Type: Breaking change label)
• No

If Yes, what's the impact:

• N/A

Pull request type

API additions

Please add the corresponding label for change this PR introduces:

• Bugfix: Type: Bug
• Feature/model/API additions: Type: Feature
• Updates to docs or samples: Type: Documentation
• Dependencies/code cleanup: Type: Maintenance

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[kfcampbell]

Thank you!