Feature Request - grpc-gateway integrated into gRPC directly

[Keith Adler]

Add a [grpc-gateway](https://github.com/grpc-ecosystem/grpc-gateway) like component that is a separate binary/process from the gRPC server. It can be deployed alongside a server of any language. This should be in-library support, and not a `grpc-ecosystem` project. That way, you wouldn't have to orchestrate a separate gateway process. To implement this, it is necessary to ensure that support is added across the language matrix, which will entail four different implementations: Core (C++), Java, Python, and Go.

## Open issues (if applicable)

gRFC Pull Request - LINK WILL GO HERE

[gRPC issue](https://github.com/grpc/grpc/issues/23979)

[grpc-gateway-repository](https://github.com/grpc-ecosystem/grpc-gateway/issues/1613)

[Keith Adler]

gRFC Pull Request -  https://github.com/grpc/proposal/pull/208

[wen...@google.com]

Thanks for submitting the proposal. Will provide initial feedback soon.

- Wenbo