feat: Nitro chat completion with image supporting local image path

[hiro-v]

Problem

• The current implementation for chat/completion with only support for base64 as image_url.url makes it hard for using curl to test out quickly. Using something like file://<absolute path> makes it easier for testing out
• While integrating with Jan app at feat: Jan RAG (PoC) jan#1023, Jan uses threads/<thread_id>/files/**.png or jpg. If Nitro supports it, it's would be very convinient

Success Criteria

• Support in Nitro in chat/completion to support image_url.url as local file path file://<absolute_path then Nitro can load image and use - https://platform.openai.com/docs/api-reference/chat/create
• (optional): Can support remote image if possible but not really necessary.

Additional context
Add any other context or screenshots about the feature request here.

[hiro-v]

It can also check: if includes https then download from remote