Add a responseMimeType to generationConfig

[zezutom]

Extension name: firestore-multimodal-genai

This feature request is for extension: _ firestore-multimodal-genai

What feature would you like to see?

I would like to configure the responseMimeType. This was recently added to the underlying library @google/generative-ai. See this commit.

How would you use it?

I want to have a possibility to instruct VertexAI to return the response in a JSON format instead of a normal textual format.