Use EventBridge integration instead of polling

[dbaggerman]

Since Buildkite now offers EventBridge integration, triggering the scaling lambda on job.* events may be more responsive than the polling method used currently - and without requiring a lambda to effectively be running 24/7.

One challenge might be preventing a large build that queues multiple jobs from firing the lambda repeatedly. However, solving that may be as simple as sticking a kinesis stream between the EventBridge and the lambda (and taking advantage of the built-in batching).

[chloeruka]

Yeah, this is a great idea! We'll try and find some time to dig into the details and figure out how feasible it'll be.