WebTransport support

[joshtriplett]

I'd love to have support for WebTransport in Trillium. WebTransport is similar to WebSocket, but rather than being a single stream, WebTransport supports connecting once and then creating multiple streams in both directions, for both unreliable datagrams and reliable streams. In particular, once connected, either the client or the server can initiate a new stream. This is incredibly handy and flexible, particularly for communications between different components of a service.