[JS][feature] Add support for 'media' in ai.chat()

[Tyg-g]

Is your feature request related to a problem? Please describe. At least one LLM family supports uploading files into conversation threads: ChatGPT. It'd be useful for me to be able to us this feature.

I assume this feature does not exist because the genkit docs on ai.chat() don't mention it.

Describe the solution you'd like Similar to the ai.generate(...) interface.

Describe alternatives you've considered x

Additional context I realize that this feature also has to be supported by the given plugin.

Note: In the case of OpenAI, the conversation threads are part of the Assistant API.