Add a web socket call concept for connecting.

Similar to HTTP and Call, the WebSocketCall is a representation of a pending HTTP request and subsequent upgrade to speak web sockets. Upon synchronous execution you are handed a WebSocket instance for synchronous writing and also pass in a WebSocketListener for async callbacks due to reading.

The API changes in this commits also generalize WebSocket such that it's agnostic to being a client or server peer.

Closes #1173.
This type and public ctor allows reuse in server context (e.g., MockWebServer soon).