Add docs for configuring GCP service account with identity pool

[csirmazbendeguz]

As someone less experienced, I would've really appreciated a bit more details in the section:

• Use dynamic credentials with the GCP provider
  • Configure GCP
    • Add a Service Account and Permissions
      • Grant External Permissions

I was getting the following error on every run:

Permission 'iam.serviceAccounts.getAccessToken' denied on resource (or it may not exist).

The solution is:

1. Go to the identity pool
2. Click Grant Access
3. Choose "Grant access using service account impersonation"
4. Select your service account
5. Set subject to organization:{organization}:project:{project}:workspace:{workspace}:run_phase:plan
6. REPEAT 1-4 & set subject to organization:{organization}:project:{project}:workspace:{workspace}:run_phase:apply
7. Confirm the subjects are set via the identity pool's "Connected service accounts" tab