Support custom embedding_function for autogen_ext.memory.chromadb

[YaakovHatam]

in the /autogen_ext/memory/chromadb.py file under the function def _ensure_initialized(self) -> None:
we can add support for custom embedding_function, the code that i changed for achieving:

        if self._collection is None:
            try:
                collection_kwargs = {
                    "name": self._config.collection_name,
                    "metadata": {"distance_metric": self._config.distance_metric},
                }
                if self._config.embedding_function is not None:
                    collection_kwargs["embedding_function"] = self._config.embedding_function
                self._collection = self._client.get_or_create_collection(**collection_kwargs)
            except Exception as e:
                logger.error(f"Failed to get/create collection: {e}")
                raise