Add WebRTC private to public example

We have an example for WebRTC private to private (e.g. two browser peers behind NATs) where we use a Circuit Relay server to exchange SDP messages to establish a direct connection between the two nodes.

As of @libp2p/webrtc@5.1.0 support has landed for WebRTC-Direct listeners in Node.js, so it would be great to add a private to public example where a Node.js listener is created and a browser dials it without needing a relay.