Add Silero VAD

[csukuangfj]

This PR supports https://github.com/snakers4/silero-vad

TODOs

• Add an alsa example to support running on embedded systems, e.g, raspberry pi
• Add a Python example
• Add Android demo
• Add ios demo
• Add an ASR example with VAD
• Add doc

Usage

Check out this PR

cd /path/to/sherpa-onnx

mkdir build
cd build
cmake ..
make -j 3
wget https://github.com/snakers4/silero-vad/raw/master/files/silero_vad.onnx

./bin/sherpa-onnx-vad-microphone --silero-vad-model=./silero_vad.onnx

[csukuangfj]

A video demo for this pull-request can be found at
https://www.bilibili.com/video/BV1A94y1s7eK/