support gRPC to JSON proxying

[jbgi]

à la https://github.com/grpc-ecosystem/grpc-gateway

[mattklein123]

cc @zinuga @louiscryan

[kyessenov]

@lizan

[mattklein123]

@zinuga @louiscryan @lizan FWIW, we would love to use this at Lyft also, so if there is any chance of getting this resourced at some point that would be pretty awesome.

[mattklein123]

cc @twoism @rodaine

[twoism]

I think this makes total sense to include. Let's talk about it when I'm in Seattle.

[lizan]

@mattklein123 Is there anyone already working on this at Lyft? If no, assign this to me, I'm working on bringing our transcoding to Envoy.

[mattklein123]

@lizan Yay. No, one is working on this. All yours. I will add you as read contributor so that I can assign you.

[mwitkow]

@lizan out of sheer curiosity: how do you transcode Protobuf field identifiers into JSON field names without the Envoy needing to know the descriptor.proto of the relevant protocol?

We had a similar problemwith our gRPC endpoints, as our proxy needed to be rebuilt whenever the schema changed, and we went with using gRPC-Web:
https://github.com/improbable-eng/grpc-web