feat: add github_repository_rulesets data source

[slymanmrcan]

Resolves #2563

Before the change?

The provider had a github_repository_ruleset resource, but there was no data source to:

• Fetch a single repository ruleset by ID
• List all existing repository rulesets for a repository

After the change?

• Added a new github_repository_ruleset data source for fetching a single ruleset by ID
• Added a new github_repository_rulesets data source for listing all rulesets for a repository
• Added acceptance tests for both data sources
• Added documentation for both data sources

Pull request checklist

• Schema migrations have been created if needed (example)
• 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!

• Yes
• No

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Note on AI Usage

AI coding assistants were used to help draft the initial code structure for both data sources. I have personally reviewed, understood, and tested all changes including:

[github-actions]

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