Add support for WebRTC sender side bandwidth estimation based on transport-cc extensions

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.88 Safari/537.36 Steps to reproduce: While Chrome supports receiver-side bandwidth estimation based on REMB and sender-side bandwidth estimation based on TransportFeedback Firefox only supports the former. The later is more flexible and modern and supporting it in Firefox would make it easier to interoperate with servers supporting only sender-side bandwidth estimation and also easier to experiment with new bandwidth estimation algorithms (f.e. NADA).