Implement OpenAI Text-to-Speech Conversion Model

[rollybueno]

Found this TODO in src/ProviderImplementations/OpenAi/OpenAiProvider.php line 44:

// TODO: Implement OpenAiTextToSpeechConversionModel.

I see there's already logic in the metadata directory to handle TTS models (if they exist in the API response). If nobody's working on this yet, I'd love to take a shot at it! Let me know..

[caseymanos]

@rollybueno Any progress? I can work on this today if needed.