Allow incoming encoded frames to be forwarded to one or more senders on other PeerConnections

[alvestrand]

This is in support of the use case outlined in w3c/webrtc-nv-use-cases#77

Experimentation has revealed two subissues here:

• It must be possible to create multiple outgoing frames from a single incoming frame
• It must be possible to adjust the metadata of the outgoing frame so that it corresponds to what is required for the outgoing PC

These will be filed as separate issues; this issue is filed to address other possible requirements.

[alvestrand]

Added an explainer that covers both this feature and other usages of encoded media manipulations in #164

[aboba]

Related: #162, #168

[dontcallmedom-bot]

This issue was mentioned in WEBRTCWG-2023-09-12 (Page 48)