Add audio support for pyav

[Azleur]

This is a proposal to add new methods to the pyav plugin API in order to expose pyav's audio read/write capabilities. Reading could look something like this:

with iio.imopen(...) as file:
    audio = file.read_audio(...)
    for audio_frame in file.read_audio():
        ... 

And writing could look something like this:

with iio.imopen(...) as file:
    file.init_audio_stream(...)
    for chunk in audio_frames:
        file.write_audio_frame(chunk)

I can contribute the code changes for this addition.

[FirefoxMetzger]

Looks good from my side. Feel free to get started with a PR and let me know if you need any help or if I should review anything.