Nomad CSI plugin GCP service account documentation addendum

### Nomad version
1.10

### Operating system and Environment details
RHEL 9

### Issue

The following guidance was provided to one of my customer by Arian Todorov:

_The CSI driver impersonates a dedicated service account just for the CSI stuff, based on its workload identity. (You can also do it with the compute one, but considering the somewhat elevated privileges like deleting disks, I thought it'd be better to have only the CSI controller which has permissions to do all this)._

Could this be added to our documentation along withe much wider guidance for using the CSP plugin on GCP with workload identity, as provided by Adrian.

### Reproduction steps
N/A

#### Expected Result
N/A

#### Actual Result
N/A

### Job file (if appropriate)
N/A