feat: Add `/webrtc` protocol (browser-to-browser)

[mxinden]

Should we introduce this in the version after https://github.com/multiformats/rust-multiaddr/pull/78? The version that includes https://github.com/multiformats/rust-multiaddr/pull/78 only would then error on /webrtc instead of interpreting it as the browser-to-browser protocol. Might prevent some confusion.

[thomaseizinger]

Should we introduce this in the version after #78? The version that includes #78 only would then error on /webrtc instead of interpreting it as the browser-to-browser protocol. Might prevent some confusion.

It is frustrating to release so many breaking changes but I don't think there is much choice now that the decision has been made.

[mxinden]

Moving to draft in order to not include in v0.18.0.

[jxs]

superseeded by #110