Consider exposing API in terms of genai_primitives.

[polina-c]

The goal of genai_primitives is to provide technology-agnostic primitive types and data structures for building Generative AI applications in Dart.

https://pub.dev/packages/genai_primitives

It may be that users will appreciate common language.

Motivation: Lossy Abstraction for Generated Responses

[domesticmouse]

Hey @polina-c, do you want to turn this into a PR?