Add Python API for keyword spotting (#576)

* Add alsa & microphone support for keyword spotting

* Add python wrapper

sherpa-onnx/python/sherpa_onnx/__init__.py

@@ -17,6 +17,7 @@ from _sherpa_onnx import (
     VoiceActivityDetector,
 )
 
+from .keyword_spotter import KeywordSpotter
 from .offline_recognizer import OfflineRecognizer
 from .online_recognizer import OnlineRecognizer
 from .utils import text2token