Add extra_body to CreateChatCompletionRequest

[jfolz]

OpenAI's Python library allows adding extra arguments to the request body via the extra_body kwarg. This is useful to, e.g., control thinking modes for Qwen. This PR adds the extra_body field to CreateChatCompletionRequest to accomplish the same.

[64bit]

This can be achieved with byot feature:

pub struct CustomCreateChatCompletionRequest {
    #[serde(flatten)]
    pub request: CreateChatCompletionRequest,
    #[serde(flatten)]
    pub extra_body: Option<Map<String, Value>>,
}