Split out API Gateway Definitions into their own library outside scalambda-core

[SwiftEngineer]

If someone is looking through the code, they might run into the ApiGateway pojos which could cause some confusion. Would be nice to split these things out and/or only inject them if the user is building a Lambda that will be invoked via an API Gateway Proxy request.

Bonus points: Define pojos for other AWS standardized requests like SNS or Cloudwatch events.