[Feature]: Support responseMimeType for Gemini (aka JSON mode?)

[Manouchehri]

The Feature

https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/gemini

Motivation, pitch

Basically, translating response_format={"type": "json_object"} for Gemini 1.5 Pro.

Twitter / LinkedIn details

https://www.linkedin.com/in/davidmanouchehri/

[Manouchehri]

Notes for future David:

curl -X POST "https://us-east4-aiplatform.googleapis.com/v1/projects/litellm-epic/locations/us-east4/publishers/google/models/gemini-1.5-pro-preview-0409:generateContent" \
    -H "Authorization: Bearer $(gcloud auth print-access-token)" \
    -H 'Content-Type: application/json' \
    -d '{
    "contents": [
        {
            "role": "user",
            "parts": [
                {
                    "text": "tell me a joke"
                }
            ]
        }
    ],
    "safety_settings": [
      {
        "category": "HARM_CATEGORY_DANGEROUS_CONTENT",
        "threshold": "BLOCK_LOW_AND_ABOVE"
      }
    ],
    "generation_config": {
      "max_output_tokens": 256,
      "response_mime_type": "application/json"
    }
}' -v