feat(github): Add GitHub Actions support

[kunaals]

Summary

Closes #2121

Adds support for GitHub Actions entities to Cartography:

• GitHubWorkflow nodes - Workflows defined in repositories
• GitHubEnvironment nodes - Deployment environments for repositories
• GitHubActionsSecret nodes - Secrets at organization, repository, and environment levels
• GitHubActionsVariable nodes - Variables at organization, repository, and environment levels

Key Features

• Uses GitHub REST API (not GraphQL) for Actions endpoints
• Supports all three levels of secrets/variables: organization, repository, and environment
• Proper rate limiting and pagination handling for REST API
• Complete schema definitions with appropriate relationships
• Secret values are never exposed (GitHub API only returns metadata)

Relationships

GitHubOrganization
    |--[RESOURCE]--> GitHubActionsSecret (org-level)
    |--[RESOURCE]--> GitHubActionsVariable (org-level)

GitHubRepository
    |--[HAS_WORKFLOW]--> GitHubWorkflow
    |--[HAS_SECRET]--> GitHubActionsSecret (repo-level)
    |--[HAS_VARIABLE]--> GitHubActionsVariable (repo-level)
    |--[HAS_ENVIRONMENT]--> GitHubEnvironment
                                |--[HAS_SECRET]--> GitHubActionsSecret (env-level)
                                |--[HAS_VARIABLE]--> GitHubActionsVariable (env-level)

Not addressed (future work)

The following items from #2121 are not implemented in this PR:

• (GitHubWorkflow)-[:USES_SECRET]->(GitHubActionsSecret) - Parsing workflow YAML files to determine which secrets are referenced by each workflow
• (GitHubWorkflow)-[:USES_ACTION]->(GitHubAction) - Modeling marketplace/external actions used by workflows
• "Last used" timestamps for secrets - GitHub's API does not expose this information

Checklist

• [x] I have read and agree to the Contribution Guidelines
• [x] If this is a code change, I have written unit and/or integration tests
• [x] I have updated the documentation

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