Allow adapting the bitrate to network/receiver constraints when using MSE

[takumif]

This issue tracks allowing sender pages to adapt the media bitrate to network/receiver constraints (e.g. bandwidth, buffer size) when using MSE to ensure a smooth playback.

One solution is to expose the remote buffer state so that the sender page can adjust the media bitrate accordingly. #126 is a strawman PR for implementing this.

Per the TPAC discussion, the first step here is to add back-pressure signaling to the Open Screen Protocol.