webrtc/: Add libp2p WebRTC private-to-private spec [tracking issue]

[p-shahi]

Now that the specification for browser to server is merged as a Working Draft: #412, we can begin the second iteration of the browser to browser aspect. See: a46919c for the beginnings.

• feat(webrtc): add WebRTC (prev. browser-to-browser) spec #497
• WebRTC: Browser to Browser js-libp2p#1462
• webrtc: private/browser-to-private/server go-libp2p#2009
• rust-libp2p https://github.com/libp2p/rust-libp2p/blob/master/ROADMAP.md#webrtc-support-browser-to-browser

[mxinden]

@p-shahi can we close here or do you want to track the concrete implementations e.g. libp2p/go-libp2p#2009 here?