ruby : VAD separately from ASR (#3518)

ruby : VAD separately from ASR (#3518)

* Add Whisper::VAD::Context

* Add test for Whisper::VAD::Context

* Add Whisper::VAD::Segment

* Add Whisper::VAD::Segments

* Add Whisper::VAD::Context#detect

* Define Whisper::VAD::Segments#each

* Define Whisper::VAD::Segment#start_time and #end_time

* Define Whisper::VAD::Segment#deconstruct_keys

* Add tests for Whisper::VAD family

* Add signatures for VAD family

* Add document on VAD in README

* Define Whisper::VAD::Segments#length

* Add test for Whisper::VAD::Segments#length

* Add signature of Segments#length

* Make vad_segments responsible to initialize VAD::Segments

* Remove meaningless argument check

* Check NULL of segments member

* Add tests for Whisper::VAD::Segments

* Initialize Whisper::VAD::Segment on .allocate

* Add tests for Whisper::VAD::Segment

* Check NULL of context member

* Add test for Whisper::VAD::Context.allocate