add RTP timestamp to metadata

[fippo]

see also #116

https://w3c.github.io/webrtc-encoded-transform/#RTCEncodedVideoFrame-interface
and
https://w3c.github.io/webrtc-encoded-transform/#RTCEncodedAudioFrame-interface
both define a timestamp as

readonly attribute unsigned long timestamp;

which by definition is the RTP timestamp (and this was changed to align with implementations).
This is different from what we have in Webcodecs

  readonly attribute long long timestamp;          // microseconds

This is hard to fix so we should do this as follows:

• add rtpTimestamp to metadata
• deprecate the existing timestamp on Encoded*Frame
• remove from implementations for 1+ release
• re-add with correct type

[aboba]

rtpTimestamp is already defined in VideoFrameCallbackMetadata so it seems logical to have it also be a property of encoded chunks in both Encoded Transform and WebCodecs.

[fippo]

@tonyherre FYI

[tonyherre]

To update on what's been happening: #173 added the long long timestamp type into the Metadata, so we at least now have that field matching the webcodecs field in both type and definition.
I created #203 to move the RTP timestamp to metadata and rename it to explicitly be rtcTimestamp - so the first bulletpoint in fippo's original comment.