record-tester: add webhook validation

We'd like record-tester to validate that the `stream.started`, `stream.idle`, `recording.started`, and `recording.ready` hooks all fire. We'd like this test to run continuously against production infrastructure. Steps are like:

1. When creating a new stream for record-tester purposes, also register all of those webhooks
2. Check that we get the webhooks back at the appropriate times. This will require a bit of infrastructure to have record-tester externally routable for the purpose of receiving those hooks back. We could also potentially use something like ngrok for this, but there's different tradeoffs associated there.