Add retrieval config support for google LLM

[chenghao-mou]

This should close #4407

Tested with the following setup.

    }
    session = AgentSession(
        ...
        llm=google.LLM(
            model="gemini-2.5-flash",
            vertexai=True,
            location="global",
            # retrieval_config=types.RetrievalConfig(
            #     lat_lng=types.LatLng(latitude=53.350140, longitude=-6.266155)
            # ),
            # or
            retrieval_config={
                "lat_lng": {
                    "latitude": 53.350140,
                    "longitude": -6.266155,
                }
            },
        ),
        ...
        tools=[
            google.tools.GoogleMaps(),
        ],
    )