Lookup project from credentials

Assuming that service credentials contain the project ID so we can look it up.

The current MR is ready to review. QA steps:

1. Use a local instance with the merge request branch
2. Enable vertex ai search if not enabled
3. Create a new search page from /admin/config/search/pages
4. Fill in the Service Account Credentials
5. Click the button 'Lookup Project'
6. Confirm you see the project ID show below the button
7. Copy the ID and save it in the Google Cloud Project ID field
8. Save the rest of the values like usual
9. Confirm the search works as expected

timozura → made their first commit to this issue’s fork.

Automatically closed - issue fixed for 2 weeks with no activity.