Enhance API-Gateway/CORS support.
Enhance API-Gateway/CORS support.

Changes:
	- Add `CORSOptions` to API struct to enable specifying the CORS headers
	- Add variadic slice to `NewMethod` to define the set of possible HTTP response codes
	- Add `method.SupportedRequestContentTypes` to scope the VTL template mappings

mweagle/Sparta,mweagle/Sparta,mweagle/Sparta,mweagle/Sparta