Add S3 event source

It would be great if either the S3 event source was added or if there was just a way to manually map an event to an event type and then can just pass in the route config like we can with the built in types

Start by tracing how the built-in event types are handled and where route configuration enters the project. Compare adding an S3 event source with supporting manual event-to-type mapping; done means one of these approaches can route S3 events through the existing configuration, with relevant tests covering the behavior.

索引モデルが issue の本文から書いたものです。