Allow custom resolver in WebSocketClientConnection

[takluyver]

SimpleAsyncHTTPClient accepts a custom resolver instance, and this can be used to make an HTTP connection to Unix sockets (see also #2671). I would like to do the same with WebSocketClientConnection. It's possible by globally configuring the resolver class, but I'd prefer to do it without global side-effects.

For more context, this is part of an effort for jupyter-server-proxy to support proxying services listening securely on Unix sockets, as well as TCP sockets (https://github.com/jupyterhub/jupyter-server-proxy/pull/337 ). This is currently working apart from Websocket support, which this PR would make possible.