Add example for multi-party video calls

[cloudwebrtc]

Create an example connect to a sfu server.
There may be mediasoup, medooze, or pions. or other SFU servers.
If we use nodejs we may need to modify https://github.com/cloudwebrtc/flutter-webrtc-server to work with it.

[shinewanna]

Bro,u run with flutter pub or source code?

[100cm]

any progress on this now?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.