feat: Add github_actions_organization_self_hosted_runners resource

[mrymam]

Resolves #2601

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Before the change?

• There is no Terraform resource to manage which repositories are allowed to create repository-level self-hosted runners at the organization level. Users must configure this setting manually via the GitHub UI (Organization Settings > Actions > Runners).

After the change?

• A new resource github_actions_organization_self_hosted_runners is added that manages the self-hosted runner creation policy using the GET/PUT /orgs/{org}/actions/permissions/self-hosted-runners API endpoints.
• Supports enabled_repositories policy: all, selected, or none.
• When set to selected, an enabled_repositories_config block allows specifying which repository IDs are permitted to create self-hosted runners.
• Import is supported via terraform import github_actions_organization_self_hosted_runners.example org_name.
• On resource deletion, the policy is reset to all (default permissive state).

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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[github-actions]

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