Generate schema for API Gateway responses

[esmarkowski]

Summary

Add Model definition to schema when building API Gateway templates. Optionally, allow JSON schema to be defined for each endpoint.

Motivation

When schema is defined in API Gateway it's possible to use the SDK to generate API clients directly from AWS. Without the schema the SDK is a useless pile of garbage.

Reference-level explanation

When using models in Jets, we should have enough information to create AWS::ApiGateway::Model. When building the CFN templates for API Gateway we could add the appropriate definition.

Type: AWS::ApiGateway::Model
Properties: 
  ContentType: String
  Description: String
  Name: String
  RestApiId: String
  Schema: Json

I'm going to take a stab at implementing this, but wanted to stub out a feature for reference.