Support for explicitly setting mime_type for vertex ai

[krrishdholakia]

Relevant PR: #8802

Proposed Input:

"image_url": {
"url": "gs://...", 
"mime_type": "image/png"
}

Concern:
This breaks the openai schema, which would make it harder to call the proxy with this specification, with OpenAI client sdk's - especially in strongly typed languages.

[krrishdholakia]

@alexanderar noted OpenAI already supports something similar for audio input -

{ type: "input_audio", input_audio: { data: base64str, format: "wav" }}

[krrishdholakia]

This is now on main - #9019

[pang-wu]

yeah, and also having the mime type within the url aligns with other modalities like image.