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: https://github.com/libp2p/specs/pull/412, we can begin the second iteration of the browser to browser aspect. See: https://github.com/libp2p/specs/pull/412/commits/a46919c557aa4501afb285ffbec8261a7a7e595b for the beginnings.

• [x] https://github.com/libp2p/specs/pull/497
• [x] https://github.com/libp2p/js-libp2p/issues/1462
• [ ] https://github.com/libp2p/go-libp2p/issues/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. https://github.com/libp2p/go-libp2p/issues/2009 here?