🔧(agents) make Silero VAD optional

Allow configuring whether a VAD model runs before calling an external ASR API.
Running VAD can save API calls (and costs) when no audible sound is detected,
but comes with the trade-off of additional computational overhead.

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to
 ### Added
 
 - ✨(summary) add dutch and german languages
+- 🔧(agents) make Silero VAD optional
 
 ### Changed