Add service account names to gcp credentials request manifest

For enabling short-lived credentials in gcp cluster using workload
identity, we need ccoctl tool to know the kubernetes service account
names from the credentials request manifest so that it can create
gcp service accounts that can be impersonated only by specific
kubernetes service accounts.

manifests/01-registry-credentials-request-gcs.yaml

@@ -20,3 +20,6 @@ spec:
     - roles/storage.admin
     - roles/iam.serviceAccountUser
     skipServiceCheck: true
+  serviceAccountNames:
+  - cluster-image-registry-operator
+  - registry