Store stack trace in a separate filed during logging (#2444)

For reasons unclear to me, if the stack trace is appended directly to
the message, the log entry will be lumped together with following logs
on GKE.

Also updated the GKE service account for Nomulus in the manifest so we
can use workload identity just for Nomulus, not other pods on the same
cluster.

jetty/kubernetes/nomulus-deployment.yaml

@@ -13,6 +13,7 @@ spec:
       labels:
         app: nomulus
     spec:
+      serviceAccountName: nomulus
       containers:
       - name: nomulus
         image: gcr.io/GCP_PROJECT/nomulus