[Feature] Add JSON schema

[vasiliadi]

https://github.com/google-gemini/generative-ai-python/releases/tag/v0.5.3
Add JSON schema support
google-gemini/deprecated-generative-ai-python#302
google-gemini/deprecated-generative-ai-python#309

https://github.com/google-gemini/cookbook/blob/main/quickstarts/JSON_mode.ipynb
https://platform.openai.com/docs/guides/structured-outputs/examples

[vasiliadi]

It is mostly needed for the identify_speakers function.