Is your feature request related to a problem? Please describe.
My current project requires access to the citation data returned from Vertex AI / Gemini. Unfortunately, this data is not exposed to the flows that the project uses.

Describe the solution you'd like
It would be nice if all data from an LLM were retained and accessible to callers. I like that Genkit has streamlined LLM responses to a useful subset, but sometimes we need a little more info. Plus, as the LLMs themselves and their services evolve, it might be that the GenerateReponse class doesn't stay in-sync with the LLM service.

Describe alternatives you've considered
I traced through the calls in my project in a debugger. I found that the generate() function, called (implicitly) by ExecutePrompt, only returns a GenerateResponse object. Looking at the resulting object, I saw that the citation metadata that was present in the response argument to the GenerateResponse constructor was not present in the resulting object.