When using the voice agent in typed code, it is suboptimal and error
prone to type the TTS voice variables in your code independently.
With this commit we are making the type exportable so that developers
can just use that and be future-proof.
Example of usage in code:
```
DEFAULT_TTS_VOICE: TTSModelSettings.TTSVoice = "alloy"
...
tts_voice: TTSModelSettings.TTSVoice = DEFAULT_TTS_VOICE
...
output = await VoicePipeline(
workflow=workflow,
config=VoicePipelineConfig(
tts_settings=TTSModelSettings(
buffer_size=512,
transform_data=transform_data,
voice=tts_voice,
instructions=tts_instructions,
))
).run(audio_input)
```
---------
Co-authored-by: Rohan Mehta <rm@openai.com>
