Support for Google Vertex AI

[djahandarie]

Validations

• [X] I believe this is a way to improve. I'll try to join the Continue Discord for questions
• [X] I'm not able to find an open issue that requests the same enhancement

Problem

Right now "Google Gemini API" is supported, but "Google Vertex AI" is not supported. This is unfortunate, as Google Vertex AI has coverage of more models than just Gemini (e.g., Claude 3.5), and is securely usable by enterprises via their existing gcloud authentication, and existing enterprise contracts with Google.

Solution

Add support for connecting to Google Vertex and selecting models from it. Most likely authenticating using Application Default Credentials would be the most reasonable way to do this. See https://cloud.google.com/vertex-ai/docs/authentication