Add support for multipart/form-data in RIE

I would love the ability to use multipart/form-data in local testing environments for my lambda functions. The official AWS API-Gateway supports this feature so I see no reason why this package shouldn't.

It could be enabled with a --flag on the startup call or something if need be.

Start by tracing how the RIE handles Lambda HTTP requests and startup flags, then compare the expected multipart/form-data behavior with AWS API Gateway. Done means local testing supports multipart/form-data for Lambda functions, with any required startup configuration documented.

索引モデルが issue の本文から書いたものです。