WebSocket sinks

[vytas7]

It seems that using sinks to match a WebSocket request is currently undocumented, and it leads to sloppy behaviour/mismatched typing, although it surprisingly works, in a sense. But the sink then receives a ws object instead of resp, so a sink method that is unaware of the possibility will likely just error out when trying to set unexpected attributes on ws.

A decision is needed how we want this to work, and what can be considered a fix, and not a breaking change.
OTOH, we have clearly documented the sink callable in add_sink() as taking req and resp parameters, so I think we can afford some redesign here. Alternatively, we could add a flag governing this behaviour.

One way to architect this is to add a ws parameter along the lines of App.add_error_handler(). However, sinks can accept arbitrary **kwargs...