Feat: Model name is configurable for the cloud function

[waziers]

By default, the cloud function will use a default model. However, you may want to test out different Gemini models are they are released. We have made the model name configurable via an environment variable.

In development, you can run the main script with a new MODEL_NAME variable:

PROJECT=XXXX LOCATION=us-central-1 VERTEX_CF_AUTH_TOKEN=$(cat ../.vertex_cf_auth_token) MODEL_NAME=XXXXX python main.py

In production, on the cloud function, you can manually set a variable in the GCP UI. Updating the variable will re-deploy the cloud function.