Migrate adaptivePtime to main spec

[dontcallmedom]

The addition of an adaptivePtime member to the RTCRtpEncodingParameters dictionary seems to have reasonable test coverage, one passing implementation, and implementation commitment from webkit, so should be ready to migrate to the main spec.

@youennf can you confirm your email to webkit-dev can be reasonably read as implementation commitment?
@jan-ivar is there any info with regard to Firefox plans for this?

[aboba]

WebKit position: https://lists.webkit.org/pipermail/webkit-dev/2021-March/031763.html
WPT test: https://wpt.fyi/results/webrtc-extensions/RTCRtpParameters-adaptivePtime.html