Switch to serde_json::Value for the response_format type

[tdwells90]

Hi,

My earlier PR was done in a bit of haste and I hadn't realized response_format was expecting an object. I thought I was expecting a string. My apologies!

So, I've changed it to Value type and it's now much better. For example:

let response_format = json!({
    "type": "json_object",
});

let request = ChatCompletionRequest::new(model, prompt).response_format(response_format);

Tested this locally and in production and working.

[dongri]