Add a dead letter queue to shortcuts.hookshot.* lambdas

[hcourt]

The hookshot shortcuts Passthrough and Github create:

• An API Gateway to receive events
• A webhook lambda to passthrough events

And expect you to provide a second lambda, that the webhook lambda passes events to.

The flow of events is then:

gateway -> webhookLambda -> myLambda

If webhookLambda fails to pass through an event, or if myLambda fails to handle an event, the event is dropped. It would be nice to provide a dead letter queue to store dropped events.