[Feature] Websocket implementation with multiplexing

Dear,

I have made a proof of concept to make an grpc-web work over a websocket where i mirrored the channel implementation of the java grpc-api. The protocol over the websocket is defined in protobuf and contains [metadata for each package like the stream id.](https://github.com/improbable-eng/grpc-web/blob/d7ac9c003bd6178380dac0bada3ef413b3b095d5/protobuf/websocketChannel.proto)

The main advantages of this implementation is:

- it is easy to implement an in proxy process
- all browsers support websockets

I was wondering if you are interested in this feature, I think it could be ported to the envoy proxy. (c++ is a long time ago for me).
[Working proof of concept](https://github.com/improbable-eng/grpc-web/pull/1084)