google-genai[minor]: Add support for json output

[juji]

Support JSON output with google-genai, by adding a json: true to instance initialization.

Example:

const model = new ChatGoogleGenerativeAI({
  model: "gemini-1.5-flash",
  json: true
})

Usage Example:

const messages = [
  new SystemMessage(`
    Translate the following English text into Italian. 

    Reply with JSON, using the following JSON schema:
    {"translation": "string"}
  `),
  new HumanMessage("hi, i am superman"),
];

const model = new ChatGoogleGenerativeAI({
  model: "gemini-1.5-flash",
  json: true
})

const parser = new JsonOutputParser()
const result = await model.pipe(parser).invoke(messages);
console.log(result)

// {"translation": "Ciao, sono Superman"}

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[bracesproul]

Thank you for pushing these updates! It would be great if you could also push up a test verifying this mode works as intended. Thank you!

[jacoblee93]

Thank you!