[EncodedTransform] Make captureTime and payloadType settable in audio frames.

This CL:
* Renames SetRtpTimestamp to SetWebRtcFrameMetadata to reflect that
  multiple metadata fields are now settable.
* Exposes captureTime in sender frames, which are locally sourced,
  using the correct epoch for them.
* Adds the new functionality to the unit test for RTCEncodedAudioFrame and
  improves the test in other aspects.
* Adds a test for peer_connection_util.h
* Adds a WPT that evaluates transmitting a modified captureTime.

Bug: 402391734
Change-Id: I136634d8b14f43125006c38a222035f8745975b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6584995
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Palak Agarwal <agpalak@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1465516}