support reading simulcast tracks with WebRTC

[aler9]

Describe the feature

Discussed in #5594

This would probably require a different track definition system.

[zapf2000]

I patched some files and made a new webplayer. I tested with h264, h265 and av1. Feel free to test
Latency looks quite good too. 3 resolutions on h264 WHIP simulcast mode ~80ms latency.