server : add Voice Activity Detection (VAD) support (#3246)

server : add Voice Activity Detection (VAD) support (#3246)

* server : add Voice Activity Detection (VAD) support

This commit adds support for Voice Activity Detection (VAD) in the
server example.

The motivation for this is to enable VAD processing when using
whisper-server.

Resolves: https://github.com/ggml-org/whisper.cpp/issues/3089

* server : add VAD parameters to usage in README.md [no ci]

This commit also adds a few missing parameters.

* server : fix conflicting short options [no ci]