Constructors for RTC Encoded Frames with metadata

[guidou]

• Specification title: Part of WebRTC encoded transform
• Specification or proposal URL (if available): Constructor frame w3c/webrtc-encoded-transform#223
• Explainer URL (if available): https://github.com/guidou/webrtc-extensions/blob/main/constructor-explainer.md
• Proposal author(s) (@-mention GitHub accounts): @guidou @palak8669
• Caniuse.com URL (optional):
• Bugzilla URL (optional):
• Mozillians who can provide input (optional): @jan-ivar

[smaug----]

@jan-ivar does this need just a spec pr review?

[jan-ivar]

LGTM. Unfortunately, RTCEncodedVideoFrame is mutable, unlike its cousin EncodedVideoChunk, so the two will likely never merge, in spite of the WebIDL comment suggesting so.

This is a copy-constructor to support client forwarding / fan-out use cases that need to modify metadata without TOKTOU issues. It's basically encoded-frame forwarding instead of packet-forwarding.