feat: add GitHub Actions hosted runner support and organization network configuration

[austenstone]

Overview

This draft PR adds support for GitHub-hosted runners and brings in the supporting organization networking pieces needed to document and configure private networking cleanly.

What this PR adds

• New github_actions_hosted_runner resource
• New github_organization_network_configuration resource
• Support for network_configuration_id on github_actions_runner_group
• Documentation updates for hosted runners, runner groups, and organization network configurations
• Example updates for hosted runners, including public_ip_enabled
• Acceptance coverage for hosted runners, runner groups, and organization network configurations

Notes

• GitHub-hosted private networking is attached through github_actions_runner_group.network_configuration_id, not directly on github_actions_hosted_runner
• public_ip_enabled is the Terraform provider field for hosted runners; the provider maps it to GitHub's API payload internally
• This branch now includes the networking/resource work that was developed on the organization network configuration branch

Validation

• go test ./github -run TestAccGithubActionsHostedRunner -count=1
• go test ./github -run TestAccGithubActionsRunnerGroup -count=1
• go test ./github -run TestAccGithubOrganizationNetworkConfiguration -count=1

[github-actions]

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