Transport: Encoder Rate Control and congestion control state

WebCodecs offers several mechanisms for fine-tuning the encoder rate, including the average bitrate parameter and per-frame QP.  But is the information available from the transport to help guide rate control?

Related: https://github.com/w3c/webcodecs/issues/270
Related: https://github.com/w3c/webcodecs/issues/580
Related: https://github.com/w3c/webtransport/pull/421