feat(github-app): allow several github apps to be used

[thomasnemer]

Summary

• Add support for configuring additional GitHub Apps to distribute API calls across multiple rate limit buckets, making the module scalable for large enterprise deployments
• New additional_github_apps variable (optional, no breaking changes) accepts a list of extra apps with id, key_base64, and optionally installation_id (all support direct values or SSM references)
• Lambda functions randomly select an app per authentication flow and thread the selection through the entire JWT → installation token → API call chain for consistency
• Installation ID resolution is optimized: primary app reuses the webhook payload, additional apps use a pre-configured installation_id when provided, falling back to an API lookup only when needed

Context

At enterprise scale with many runner configurations, all Lambda functions (scale-up, scale-down, pool, job-retry) share a single GitHub App's rate limit bucket (15,000 req/hour on GitHub Enterprise Cloud). This ceiling cannot be raised per-app — the only way to scale is to spread load across multiple apps. This change makes the effective limit N × 15,000 req/hour, without introducing breaking changes.

Closes #5037