Add support for Google Vertex AI provider with application-default and service account key login

Google models can be accessed through two different methods. Either with an API key or through Vertex AI.
Organisations use Vertex AI as it gives better control and higher rate limits usually.
To authenticate through Vertex AI one can use a service account key file (not really recommended in most cases) or use application default credentials (logging in with the gcloud cli, no need to use service account key files that are a security risk).