AudioStream API

Hi @PaulStoffregen,

We're exploring adding an AudioStream API to Arduino that extends the current Stream type. The draft interface can be found here: [api/Audio/AudioStream.h](https://github.com/arduino/ArduinoCore-API/blob/master/api/Audio/AudioStream.h). We would appreciate any feedback you have on it.

We've taken a look at the Teensy Core's [AudioStream type](https://github.com/PaulStoffregen/cores/blob/master/teensy3/AudioStream.h) and noticed it doesn't have any public interfaces to access the raw sample data. Was this done for performance reasons?

Uses cases we are thinking about include:
- [reading a file from SD card and playing back via I2S or DAC](https://github.com/arduino/ArduinoCore-API/blob/master/api/Audio/examples/WaveToI2S/WaveToI2S.ino)
- [reading data from a I2S or ADC microphone and saving to SD card](https://github.com/arduino/ArduinoCore-API/blob/master/api/Audio/examples/I2SToWave/I2SToWave.ino)

首先阅读 api/Audio/AudioStream.h 以及 Audio 示例 WaveToI2S/WaveToI2S.ino 和 I2SToWave/I2SToWave.ino，然后对比链接的 Teensy AudioStream 类型。该 issue 要求对接口草案以及现有设计背后的原因提供反馈，因此要完成它，需要就 API 方向达成一致，而不是实施一个明确定义的代码更改。

由索引模型根据 Issue 内容生成。