VertexAISearchRetriever does not support "search_kwargs"

[Freezaa9]

Hi,

GoogleVertexAISearchRetriever does not support "search_kwargs".
Would be nice to support search_kwargs to be able to create code when you can easily switch vector store.
Usually in vector store in langchain you don't specify the number of document to retrieve when creating the retriever but afterward by setting "k"

retriever = VertexAISearchRetriever(    
project_id=PROJECT_ID,
    location_id=LOCATION_ID,

    data_store_id=DATA_STORE_ID,
    max_documents=10,
    engine_data_type=1,
)
retriever.search_kwargs = {"k": 5}
relevant_documents = retriever.invoke(query)

ValueError: "VertexAISearchRetriever" object has no field "search_kwargs"

Thanks !

[lkuligin]

Would you be open to send a PR that fixes this, please?