Split flutter-webrtc pacakge to interface/native/web

If we split `flutter_webrtc` into three packages, we will be able to support `flutter native`, `flutter web`, and `dart web` at the same time.

`flutter_webrtc_platform_interface` (pure dart package)
`flutter_webrtc` (flutter package)
`flutter_webrtc_web` (pure dart package)

Just like `video_player`
https://github.com/flutter/plugins/tree/master/packages/video_player