openai[patch]: openai proxy added to base embeddings (#24539)

- [ ] **PR title**: "langchain-openai: openai proxy added to base
embeddings"

- [ ] **PR message**: 
    - **Description:** 
    Dear langchain developers,
You've already supported proxy for ChatOpenAI implementation in your
package. At the same time, if somebody needed to use proxy for chat, it
also could be necessary to be able to use it for OpenAIEmbeddings.
That's why I think it's important to add proxy support for OpenAI
embeddings. That's what I've done in this PR.

@baskaryan

---------

Co-authored-by: karpov <karpov@dohod.ru>
Co-authored-by: Bagatur <baskaryan@gmail.com>

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

@@ -282,12 +282,44 @@ class OpenAIEmbeddings(BaseModel, Embeddings):
             "default_headers": values["default_headers"],
             "default_query": values["default_query"],
         }
+
+        if values["openai_proxy"] and (
+            values["http_client"] or values["http_async_client"]
+        ):
+            openai_proxy = values["openai_proxy"]
+            http_client = values["http_client"]
+            http_async_client = values["http_async_client"]
+            raise ValueError(
+                "Cannot specify 'openai_proxy' if one of "
+                "'http_client'/'http_async_client' is already specified. Received:\n"
+                f"{openai_proxy=}\n{http_client=}\n{http_async_client=}"
+            )
         if not values.get("client"):
+            if values["openai_proxy"] and not values["http_client"]:
+                try:
+                    import httpx
+                except ImportError as e:
+                    raise ImportError(
+                        "Could not import httpx python package. "
+                        "Please install it with `pip install httpx`."
+                    ) from e
+                values["http_client"] = httpx.Client(proxy=values["openai_proxy"])
             sync_specific = {"http_client": values["http_client"]}
             values["client"] = openai.OpenAI(
                 **client_params, **sync_specific
             ).embeddings
         if not values.get("async_client"):
+            if values["openai_proxy"] and not values["http_async_client"]:
+                try:
+                    import httpx
+                except ImportError as e:
+                    raise ImportError(
+                        "Could not import httpx python package. "
+                        "Please install it with `pip install httpx`."
+                    ) from e
+                values["http_async_client"] = httpx.AsyncClient(
+                    proxy=values["openai_proxy"]
+                )
             async_specific = {"http_client": values["http_async_client"]}
             values["async_client"] = openai.AsyncOpenAI(
                 **client_params, **async_specific