setMetadata for redundant relay PCs

[palak8669]

This is in support of the use case 3.2.2 Low latency broadcast with fanout.

The use case mentions using P2P relays but because of the unreliability of the relay PCs, we want to support adding redundant input relay PCs to a peer and achieve glitch-free failover in case one of the input PCs fails. As mentioned in the interim WEBRTCWG-2023-07-18 (Page 35) for this to be possible, we need to modify the Metadata of encoded frames.

The following attributes would have to be modified:

• RTPTimestamp (for both audio and video)
• frameId (only video)
• dependencies (only video)

[guidou]

cc @jan-ivar @youennf @alvestrand

[palak8669]

This also requires removing a single PC restriction for a reader/writer pair which has been mentioned in @alvestrand's write-up on encoded data access: https://lists.w3.org/Archives/Public/public-webrtc/2022Aug/0032.html

[dontcallmedom-bot]

This issue was mentioned in WEBRTCWG-2023-09-12 (Page 50)

[jan-ivar]

This seems like an implementation-detail in service of an existing use case, not a use case in itself. I suggest closing.