This PR implements `Transport` for WebRTC for browsers by using web-sys. Only the `webrtc-direct` spec is implemented. The `webrtc` spec for connecting two browsers with each other is left to a future PR.
Related: https://github.com/libp2p/specs/issues/475.
Related #2617.
Supersedes: #4229.
Pull-Request: #4248.
Co-authored-by: Thomas Eizinger <thomas@eizinger.io>
