[Feature Request] audio_format Azure speech

I saw that in Azure Engine, it only support sample from 16khz -48khz. I expect that RealtimeTTS support 8khz because of 8khz is used in VOIP(Asterisk).

```
 "riff-8khz-16bit-mono-pcm": tts.SpeechSynthesisOutputFormat.Riff8Khz16BitMonoPcm
```


