Support custom payload exchange

[micHar]

Right now we only expose exchanging messages via the same methods that the original okhttp WebSocketListener uses, i.e. String and ByteString. We could introduce a mechanism similar to Retrofit Converters to exchange custom strongly typed messages. We need to consider that the request types and response type might be different, too. Also this might be a good place to introduce the EnvelopeJsonAdapter. Basically when building the RxWebSocket we would provide the Converter Factory. And then listening to a specific type of responses would try to search for an appropriate converter to deal with the payload. Same with sending requests. Not sure if it makes sense without codegen, too.