Add AudioFolder packaged loader
will close  #3964

AudioFolder is almost identical to ImageFolder except for inferring labels is not the default behavior (`drop_labels` is set to True in config), the option of inferring them is preserved though.

The weird thing is happening with the `test_data_files_with_metadata_and_archives` when `streaming` is `True`. Here is the log from the CI:
```

../.pyenv/versions/3.6.15/lib/python3.6/site-packages/datasets/features/audio.py:237: in _decode_non_mp3_path_like
    array, sampling_rate = librosa.load(f, sr=self.sampling_rate, mono=self.mono)
../.pyenv/versions/3.6.15/lib/python3.6/site-packages/librosa/util/decorators.py:88: in inner_f
    return f(*args, **kwargs)
../.pyenv/versions/3.6.15/lib/python3.6/site-packages/librosa/core/audio.py:176: in load
    raise (exc)
../.pyenv/versions/3.6.15/lib/python3.6/site-packages/librosa/core/audio.py:155: in load
    context = sf.SoundFile(path)
../.pyenv/versions/3.6.15/lib/python3.6/site-packages/soundfile.py:629: in __init__
    self._file = self._open(file, mode_int, closefd)
../.pyenv/versions/3.6.15/lib/python3.6/site-packages/soundfile.py:1184: in _open
    "Error opening {0!r}: ".format(self.name))
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

err = 72
prefix = "Error opening <zipfile.ZipExtFile name='audio_file.wav' mode='r' compress_type=deflate>: "

    def _error_check(err, prefix=""):
        """Pretty-print a numerical error code if there is an error."""
        if err != 0:
            err_str = _snd.sf_error_number(err)
>           raise RuntimeError(prefix + _ffi.string(err_str).decode('utf-8', 'replace'))
E           RuntimeError: Error opening <zipfile.ZipExtFile name='audio_file.wav' mode='r' compress_type=deflate>: Error in WAV file. No 'data' chunk marker.
```
I hadn't been able to reproduce this locally until I created the same test environment (I mean with `pip install .[tests]`) with python3.6. The same env but with python3.8 passes the test! I didn't manage to figure out what's wrong, I also tried simply to replace the test wav file and still got the same error. Versions of `soundfile`, `librosa` and `libsndfile` are identical. Might it be something with zip compression? Sounds weird but I don't have any other ideas... 
 
TODO:

- [x] align with #4622
- [x] documentation
- [x] tests for AutoFolder?