feat: pass extra_body through to LiteLLM acompletion

[AshokSaravanan222]

Purpose
Allow arbitrary extra_body parameters (e.g. cached_content) to be forwarded into the LiteLLM call. Useful for context caching in Gemini models (docs).

Example usage

import os
from agents import Agent, ModelSettings
from agents.extensions.models.litellm_model import LitellmModel

cache_name = "cachedContents/34jopukfx5di"  # previously stored context

gemini_model = LitellmModel(
    model="gemini/gemini-1.5-flash-002",
    api_key=os.getenv("GOOGLE_API_KEY")
)

agent = Agent(
    name="Cached Gemini Agent",
    model=gemini_model,
    model_settings=ModelSettings(
        extra_body={"cached_content": cache_name}
    )
)

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