Create Audio feature
Create `Audio` feature to handle raw audio files.

Some decisions to be further discussed:
- I have chosen `soundfile` as the audio library; another interesting library is `librosa`, but this requires `soundfile` (see [here](https://github.com/librosa/librosa/blob/main/setup.cfg#L53)). If we require some more advanced functionalities, we could eventually switch the library.
- I have implemented the audio feature as an extra: `pip install datasets[audio]`. For the moment, the typical datasets user uses only text datasets, and there is no need for them for additional package requirements for audio/image if they do not need them.
- For tests, I require audio dependencies (so that all audio functionalities are checked with our CI test suite); I exclude Linux platforms, which require an additional library to be installed with the distribution package manager
  - I also require `pytest-datadir`, which allow to have (audio) data files for tests
- The audio data contain: array and sample_rate.
- The array is reshaped as 1D array (expected input for `Wav2Vec2`).

Note that to install `soundfile` on Linux, you need to install `libsndfile` using your distribution’s package manager, for example `sudo apt-get install libsndfile1`.

## Requirements Specification
- Access example with audio loading and resampling:
  ```python
  ds[0]["audio"]
  ```
- Map with audio loading & resampling:
  ```python
  def preprocess(batch):
       batch["input_values"] = processor(batch["audio"]).input_values
       return batch

  ds = ds.map(preprocess)
  ```
- Map without audio loading and resampling:
  ```python
  def preprocess(batch):
       batch["labels"] = processor(batch["target_text"]).input_values
       return batch

  ds = ds.map(preprocess)
  ```
- Additional requirement specification (see https://github.com/huggingface/datasets/pull/2324#pullrequestreview-768864998): Cast audio column to change sampling sate:
  ```python
  ds = ds.cast_column("audio", Audio(sampling_rate=16_000))
  ```