Filter out silences

[yujonglee]

We are already doing something like this, but not very reliable.
Filtering our silence is very important since Whisper tends to hallucinate a lot.

[yujonglee]

Need #664 to be merged, and move to silero-based-predictor

[yujonglee]

https://docs.rs/voice_activity_detector/latest/voice_activity_detector/

We need both acc buffer size + current prob to decide the chunk