Support `urn:ietf:params:rtp-hdrext:ntp-64` RTP extension (RFC 6051)

Overview

Support urn:ietf:params:rtp-hdrext:ntp-64 (RFC 6051) on the consumer leg. This is the only in band capture time that some RTP stacks (such as GStreamer) understand natively, which is more efficient than relying on RTCP Sender Reports.

This is task 12 of #1881 which has been moved to a separate issue since it's not really mandatory to accomplish with the purpose of #1881.

Notes

• We don't want to do this before #1126 is done because we don't want to add yet another extension in RTP packets that not even WebRTC browsers understand. So let's wait until PR #1783 by @jiyeyuran is merged.