insertable streams: expose payload type

exposes the payload type in the encoded audio/video frame metadata
spec PR:
  https://github.com/w3c/webrtc-encoded-transform/pull/117

BUG=webrtc:13077

Change-Id: I17340aec0b18637d7c1f3222add496a8aeb346ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3118568


Reviewed-by: Harald Alvestrand <hta@chromium.org>
Reviewed-by: Tony Herre <toprice@chromium.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/main@{#918748}