Update MixedReality-WebRTC and small change in signaling protocol

[tana]

• Updated MixedReality-WebRTC library to ver 2.0.
  • Some code change is needed due to API change.
  • The library is now installed using Unity package manager.
  • See https://microsoft.github.io/MixedReality-WebRTC/manual/unity/helloworld-unity-importwebrtc.html and https://microsoft.github.io/MixedReality-WebRTC/manual/migration-guide.html
• ICE (STUN) server address is now specified by signaling server.