Rewrite the flutter webrtc server using golang.

[cloudwebrtc]

• Use flutter web instead of React.js.
• Support the built-in turn/stun server uses pion/webrtc stack.
• More demos, such as multi-party video conference, live broadcast, etc.
• Add cross-platform server binaries.

Have other ideas? Please leave a comment below

[cloudwebrtc]

https://github.com/cloudwebrtc/flutter-webrtc-server/pull/28