feat: Add github_actions_hosted_runner resource

[austenstone]

Adding support for GitHub-hosted runners

Before the change?

• No support for managing GitHub-hosted runners via Terraform
• Organizations had to manually configure hosted runners through the GitHub web UI
• No way to automate hosted runner lifecycle (create, update, delete) as code

After the change?

• New github_actions_hosted_runner resource for managing GitHub-hosted runners
• Import support for existing hosted runners
• Comprehensive test coverage.
• Complete documentation in website/docs/r/actions_hosted_runner.html.markdown
• Example Terraform configuration in examples/hosted_runner/

Pull request checklist

• Schema migrations have been created if needed (example) - N/A, this is a new resource
• 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!

• No

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