[Vertex AI] Add error message for Firebase ML API not enabled

[andrewheard]

If a developer tries to use the Vertex AI SDK without going through the Firebase Console setup flow then they will receive an error saying:

Firebase ML API has not been used in project 1234567890 before or it is disabled...

Since it may not be clear that the firebaseml.googleapis.com API is needed for Vertex AI, added a more detailed log message with direct links to the Firebase Console page:

The Vertex AI for Firebase SDK requires the Firebase ML API firebaseml.googleapis.com to be enabled for your project. Get started in the Firebase Console (https://console.firebase.google.com/project/my-project-id/genai/vertex) or verify that the API is enabled in the Google Cloud Console (https://console.developers.google.com/apis/api/firebaseml.googleapis.com/overview?project=my-project-id).

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