github-events: Add WEBHOOK_ID and REQUESTED_ONLY_WEBHOOK_ID filters

[wlynch]

Adds an option to only listen to check_run only webhook events for specific webhooks.

In our current deployment, we have multiple webhooks that funnel into the trampoline from multiple sources. Some are repo-level webhooks, some are org-level webhooks. We want to move to a model where we route all of these through a GitHub App, but to do this without downtime or sending duplicate events is tricky. This aims to solve this by restricting a specific webhook to only events that are unique to GitHub Apps - requested check_{run,suite} events.

This will allow us to rollout the new webhook but only pass through these specific events. Once we verify they work, then we can ignore the old webhooks and fully trust the new webhook all in one deployment.

1. WEBHOOK_ID=nil REQUESTED_ONLY_WEBHOOK_ID=nil | Receive all events
2. WEBHOOK_ID=nil REQUESTED_ONLY_WEBHOOK_ID=1 | Receive all events except from webhook 1, and only request checks from webhook 1
3. Start sending events from webhook 1
4. WEBHOOK_ID=1 REQUEST_ONLY_WEBHOOK_ID=nil | Only receive events from webhook 1
5. Stop sending events from all other webhooks.

If there are any problems, we can safely rollback the release since the configs are contained here.