TTS/STT: Text-to-Speech using Gemini in Unified API

[kartpop]

Is your feature request related to a problem? Please describe.
Currently the unified API only supports does not support text-to-speech use cases.
Since TTS/STT use cases are on priority, extending the llm/call endpoint to support Gemini (and others) as well audio as an extra modality as well.

Describe the solution you'd like
Start adding Gemini-2.5-Pro-TTS as a TTS provider to the unified API. Integrate others simultaneosuly.

Additional context
Gemini Docs

Solution Doc