Add “connected” hook for streaming clients

Currently, grpc-web has data, status, error, and end hooks for streaming clients. In my use case, I need to keep track of whether or not I'm currently holding a functional streaming connection with my backend, and I want to derive a connection status (connected/disconnected) from that.

If the backend is unreachable, the error or end hook gets called, but it's impossible to determine whether the connection has actually been established until I actually receive a message (or the error/end hook gets called). This could be solved by providing another hook that is called once the connection has successfully been established.

Workarounds consist in using a timeout after which, lacking any error/end hook call, I consider the streaming connection established, but that's cumbersome and needn't necessarily reflect the truth, or in modifying the backend to send some sort of ping message right after having accepted a client connection, but that of course depends on the ability to modify the backend and likewise is rather ugly.