Expose captureTimestamp to RTCEncodedAudioFrame and RTCEncodedVideoFrame

[murillo128]

By exposing the absCaptureTime header extension values we could easily sync audio and video frames and datachannel messages:

https://w3c.github.io/webrtc-extensions/#dom-rtcrtpcontributingsource-capturetimestamp

Use case: https://w3c.github.io/webrtc-nv-use-cases/#auction

[fippo]

Already exposed in https://wicg.github.io/video-rvfc/, it would be useful here too

[guidou]

cc @tonyherre