Show executing in pendingTasks toString output (elastic#130659)

A trivial change to make it easier to tell which task is executing. It
is also consistent with the toXContent output.

Author: ywangd

server/src/main/java/org/elasticsearch/action/admin/cluster/tasks/PendingClusterTasksResponse.java

@@ -49,6 +49,7 @@ public String toString() {
                 .append(pendingClusterTask.getSource())
                 .append("/")
                 .append(pendingClusterTask.getTimeInQueue())
+                .append(pendingClusterTask.executing() ? "[executing]" : "")
                 .append("\n");
         }
         return sb.toString();