LibWebSocket+RequestServer: Add a WebSocketImpl using libcurl

This implementation can be better improved in the future by ripping
out a lot of the manual logic in LibWebSocket and rely on libcurl to
parse our message payloads. But for now, this uses the 'raw mode' of
curl websockets in connect-only mode to allow for somewhat seamless
integration into our event loop.

Libraries/LibWebSocket/Impl/WebSocketImpl.h

@@ -27,6 +27,8 @@ public:
     virtual bool eof() = 0;
     virtual void discard_connection() = 0;
 
+    virtual bool handshake_complete_when_connected() const { return false; }
+
     Function<void()> on_connected;
     Function<void()> on_connection_error;
     Function<void()> on_ready_to_read;