Add onAudioStart and onAudioStop events to WaveStreamPlayer

[Stevenic]

I need to know when playback of the generated audio stream finishes so I updated my copy of the WaveStreamPlayer class to include onAudioStart and onAudioStop events. It might be useful to just add these to that class. Here's the changes I made:

[khorwood-openai]

This is helpful! I think the intention would be to open source this library; so let me look into that and update this when there's a decision there.