Add option to intoduce system message to Google germini Model

[okoliechykwuka]

By default, google Gemini doesn't support system messages, can we add that to the Gemini model when using it to power Agents.

From Langchain:

from langchain_core.messages import HumanMessage, SystemMessage

model = ChatGoogleGenerativeAI(model="gemini-pro", convert_system_message_to_human=True)
model(
    [
        SystemMessage(content="Answer only yes or no."),
        HumanMessage(content="Is apple a fruit?"),
    ]
)

[joaomdmoura]

Why not just add it to the task prompt? if it's being converted into a human message already