Support Gemini API's `response_json_schema` for structured output

[mattgiles]

Description

It would be great if use of Gemini models made use of their first class support for json schema -- as it seems like it boasts significantly improved performance for complex structured output.

References

https://blog.google/technology/developers/gemini-api-structured-outputs/

[DouweM]

Already being implemented in #3357!