AudioMixer is missing (but implementation exists :))

### Select which package(s) are affected \@livekit/rtc-node ### Describe the bug I found https://github.com/livekit/node-sdks/tree/brian/audio-mixer with a working implementaiton already, what's blocking it from bringin it to main? :) Thanks Simon ### Reproduction - ### Logs ```shell ``` ### System Info ```shell - ``` ### LiveKit server version LiveKit cloud ### Severity serious, but I can work around it ### Additional Information _No response_