Extensible websocket events

[eliihen]

Right now extensibility is extremely basic, basically only providing the same features as would be provided by the CLI options.

It may be a good idea to add extensibility of the websocket event handling to the core. This way it would be possible to use wurl as a library to implement handling WS extensions like socket.io.

See https://ws-rs.org/docs for the available events.

Some psuedo code:

let options = wurl::util::Options {
  on_message: |client: Client, msg: Message| -> Result<()> {
    // ... Do things
    Ok()
  }
};
wurl::network::connect(&options);