openai[patch]: add support for extra_body (#23404)

**Description:** Add support passing extra_body parameter

Some OpenAI compatible API's have additional parameters (for example
[vLLM](https://docs.vllm.ai/en/latest/serving/openai_compatible_server.html#extra-parameters))
that can be passed thought `extra_body`. Same question in
https://github.com/openai/openai-python/issues/767

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libs/partners/openai/langchain_openai/llms/base.py

@@ -137,6 +137,9 @@ class BaseOpenAI(BaseLLM):
     http_async_client: Union[Any, None] = None
     """Optional httpx.AsyncClient. Only used for async invocations. Must specify 
         http_client as well if you'd like a custom client for sync invocations."""
+    extra_body: Optional[Mapping[str, Any]] = None
+    """Optional additional JSON properties to include in the request parameters when
+    making requests to OpenAI compatible APIs, such as vLLM."""
 
     class Config:
         """Configuration for this pydantic object."""
@@ -222,6 +225,9 @@ class BaseOpenAI(BaseLLM):
         if self.max_tokens is not None:
             normal_params["max_tokens"] = self.max_tokens
 
+        if self.extra_body is not None:
+            normal_params["extra_body"] = self.extra_body
+
         # Azure gpt-35-turbo doesn't support best_of
         # don't specify best_of if it is 1
         if self.best_of > 1: