fix(model): image_url changed to object from string as specified in OpenAI API spec

[MbBrainz]

I noticed the request Exception below and went searching.

As discussed in the openai community forum here The image_url should be an object and not a string. In the current implementation, the image_url will be set as a string and this will always result in the following response from the openai api:

RequestFailedException(message: Invalid type for 'messages[1].content[0].image_url': expected an object, but got a string instead

In the forum discussion, it is mentioned that the api specs were outdated, but its not anymore as specified under the image tab at the openai chat completion api spec

[MbBrainz]

@anasfik @signmotion

[MbBrainz]

Fixes #191

[RuABraun]

I think this also needs a change

openai/lib/src/core/models/chat/sub_models/choices/sub_models/sub_models/content.dart

Line 62 in 8cbc061

type: 'image_base64',

The type needs to be "image_url". I found that to work and it also matches the docs