Authentication via VertexAI

[BassCoder2808]

Is your feature request related to a problem? Please describe.
When we try to use Gemini LLM we only have option to pass it as an API KEY, what if we have a json file through which we want to connect to our LLM Project?

Describe the solution you'd like
In gemini_chat.py inside the else part, we can have a solution of passing the path to the json file through which we will be connecting to the Gemini LLM project

The code for connection can look something like this

`
if "api_key" in config or os.getenv("GOOGLE_API_KEY"):
import google.generativeai as genai

        genai.configure(api_key=config["api_key"])
        self.chat_model = genai.GenerativeModel(model_name)
    else:
        # Authenticate using VertexAI
        import google.auth
        import vertexai
        from vertexai.generative_models import GenerativeModel, Part
        os.environ['GOOGLE_APPLICATION_CREDENTIALS'] = PATH
        credentials, _ = google.auth.default()
        vertexai.init()
        self.chat_model = GenerativeModel("gemini-pro")

`