Image support in `NvidiaChatGenerator`

[anakin87]

We want to make this component support multimodal user messages.

The component inherits from the OpenAIChatGenerator, but unfortunately Nvidia seems to use a different format for passing images, so it does not work out of the box

Relevant info:

• https://docs.api.nvidia.com/nim/reference/multimodal-apis
• https://build.nvidia.com/models?q=multimodal
• https://build.nvidia.com/google/gemma-3-27b-it

[anakin87]

I was testing the integration in a wrong way.

It works. Tested with

• mistralai/mistral-medium-3-instruct
• microsoft/phi-4-multimodal-instruct
• meta/llama-3.2-11b-vision-instruct