v1.8.0: Audio 代码重构与低功耗优化 (#943)

* Reconstruct Audio Code

* Remove old IoT implementation

* Add MQTT-UDP documentation

* OTA升级失败时，可以继续使用

main/audio/codecs/dummy_audio_codec.h

@@ -0,0 +1,16 @@
+#ifndef _DUMMY_AUDIO_CODEC_H
+#define _DUMMY_AUDIO_CODEC_H
+
+#include "audio_codec.h"
+
+class DummyAudioCodec : public AudioCodec {
+private:
+    virtual int Read(int16_t* dest, int samples) override;
+    virtual int Write(const int16_t* data, int samples) override;
+
+public:
+    DummyAudioCodec(int input_sample_rate, int output_sample_rate);
+    virtual ~DummyAudioCodec();
+};
+
+#endif // _DUMMY_AUDIO_CODEC_H