Feature Request: Support extra_body Parameter for Advanced Model Features

[wqj97]

Background
vLLM currently supports various model features through configuration parameters, but lacks support for passing additional model-specific parameters through extra_body, which is particularly important for features like structured output.
https://github.com/vllm-project/vllm/blob/v0.6.0/vllm/engine/arg_utils.py#L276

Current OpenAI implementation

completion = client.chat.completions.create(
    model="gpt-3.5-turbo",
    messages=[{"role": "user", "content": "Generate a user profile"}],
    extra_body={
        "guided_json": Test.schema_json,
        "guided_decoding_backend": "lm-format-enforcer"
    }
)

Proposed implementation

resp, err := integrations.LLMClient.Client.CreateChatCompletion(
		ctx,
		openai.ChatCompletionRequest{
			Model: "...",
			Messages: []openai.ChatCompletionMessage{
				...
			},
			ExtraBody: map[string]any{
				....
			},
		},
	)

[nagar-ajay]

This can be useful incase of other engines as well e.g. Nvidia NIMs - https://build.nvidia.com/nvidia/nv-embedqa-e5-v5?snippet_tab=Python