Dominant Speaker support

Problem to solve

Add support for the dominant speaker API

Proposal

Add iOS and Android support for dominant speaker API, including surfacing dominantSpeakerChanged events, and adding a dominant speaker property to the Dart Room object.

And the ConnectOptions need to be extended to support the enableDominantSpeaker property.

More information

• https://www.twilio.com/docs/video/detecting-dominant-speaker