flvmux: Add support for enhanced rmtp multitrack video

Latest version of the enhanced rtmp spec has added support for sending multitracks ver the same rtmp stream track, allowing to create multiple renditions of the same video track or carry multiple audio tracks in different languanges:

https://veovera.org/docs/enhanced/enhanced-rtmp-v2

This is already supported by OBS: https://github.com/obsproject/obs-studio/pull/10618

Twicth: https://help.twitch.tv/s/article/multiple-encodes?language=en_US https://help.twitch.tv/s/article/enhanced-broadcasting-getting-started?language=en_US

and Dolby Millicast: https://docs.dolby.io/streaming-apis/docs/obs-enhanced-broadcasting-multitrack-video