[webrtc-pc] Control surface for header extensions (#2243)

henbos has just created a new issue for https://github.com/w3c/webrtc-pc:

== Control surface for header extensions ==
For codecs we have getCapabilities() for figuring out what is supported by the browser, setCodecPreferences() for controlling what to negotiate, and getParameters() to see what was actually negotiated. In addition, getStats() can tell you more details (such as if multiple codecs were negotiated, which one is being used right now).

Should we have a similar API for header extensions? There is already getParameter()'s [headerExtensions](https://w3c.github.io/webrtc-pc/#dom-rtcrtpparameters-headerextensions) for reading what RTP header extensions were negotiated, but there is no way to detect what is supported without trial and error, and worse yet, if you want to control which ones are negotiated you have to munge SDP.

I don't expect SDP munging to go away if we don't have APIs for things like this.

@jonex

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2243 using your GitHub account