Update ws example

[mattlink]

Adds another endpoint to examples/basic_ws.py which illustrates how to read data from the websocket.

It is not clear from the existing example (or docs, currently) that you can specify a data argument to the handler function to effectively get the contents of ws.receive(). There are other hidden arguments which seem to correspond to these annotations

[jph00]

Thanks - this is a good idea. I guess it also probably needs something on the page demonstrating its use?

[jph00]

@mattlink just bumping this...

[Kvit]

Another issue that I think should be covered in docs/examples is how to pass a variable from the web socket URL path. It is done via ws param.
Here is an example snippet

from starlette.websockets import WebSocket

@rt.ws('/wscon/{session_id}', conn=on_connect, disconn=on_disconnect)
async def ws_func(ws:WebSocket, data, send):
    session_id = ws.path_params.get("session_id")
    #... rest of the code

NB: you can pass URL path param to on_connect(), on_disconnect() functions as well.