feat(silero): add Silero-vad backend

[mudler]

Description

This PR adds the silero VAD backend to enable voice activity detection (https://en.wikipedia.org/wiki/Voice_activity_detection).

Example modelfile:

name: vad
backend: silero-vad
parameters:
  model: silero_vad.onnx

Notes for Reviewers

This has been extracted as part of #3722

#1126

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