Adding additional generic to WebSocketSubject

[jkossis]

Both WebSocketSubject<T> and it's creator function webSocket<T> currently only take one generic, for distinguishing the type that the internal Subject holds. As it currently stands, .next(data: T) and .subscribe((data: T) => { ... }) force the data to be of the same type.

However, data emitted to the web socket server could be of a different shape/form than the data received from the web socket server. What are people's thoughts on extending this functionality to include a 2nd generic, perhaps of type R, to allow for stronger typing on non-symmetrical communication?

[cartant]

Related #5381 and #5936.