Add types for lambda event and context

[wulfmann]

It would be nice to be able to map in the api gateway types for event and context to use in typescript handlers.

[wulfmann]

Might as well include response types as well

[wulfmann]

This already exists in definitely typed. Let’s just add a note in the documentation.

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/aws-lambda/trigger/api-gateway-proxy.d.ts