Implement Vertex AI authentication modes and configuration in UI

- Updated index.html to include options for Vertex AI authentication modes (Express and Full).
- Enhanced openai.js to manage Vertex AI settings, including project ID, region, and service account JSON.
- Added validation and handling for service account JSON in the backend.
- Modified API request handling in google.js to support both authentication modes for Vertex AI.
- Updated secrets.js to include a key for storing Vertex AI service account JSON.
- Improved error handling and user feedback for authentication issues.

public/scripts/secrets.js

@@ -44,6 +44,7 @@ export const SECRET_KEYS = {
     SERPER: 'api_key_serper',
     FALAI: 'api_key_falai',
     XAI: 'api_key_xai',
+    VERTEXAI_SERVICE_ACCOUNT: 'vertexai_service_account_json',
 };
 
 const INPUT_MAP = {