Service Account attached to GCP VMs isn't supported as authentication method for GCS external links

Instead of using a Service Account's JSON credentials and passing it to authenticate a GCS external link, the user has the option to attach the Service Account to a GCP VM, and allow the link to pick up the authentication through the VM itself, this is the most recommended approach when Couchbase is deployed on a GCP VM machine.

This is similar to instance profile (AWS EC2) and managed identity (Azure VM) authentications where the authentication entity is attached to the AWS EC2 or Azure VM, and no credentials need to be passed to the link.