Add additional fields (revised_prompt, b64_json) to image response

[spantaleev]

Ref:

• https://platform.openai.com/docs/api-reference/images/object
• https://github.com/openai/openai-openapi/blob/440c1718f0ea39d32d3735018cbcd6785a93db79/openapi.yaml#L10199-L10208

As per the OpenAI spec, these fields are always non-optional and always returned as a potentially-empty string.

In practice, other providers (like LocalAI) only return a b64_json field if response_format = b64_json. There's neither a url field, nor a revised_prompt.

For maximum compatibility, url was also made an Option<String>.