Add CancelWorkflowTaskAttributes

protos/local/core_interface.proto

@@ -65,6 +65,7 @@ message WFActivationJob {
         TimerFiredTaskAttributes timer_fired = 2;
 
         QueryWorkflowJob query_workflow = 3;
+        CancelWorkflowTaskAttributes cancel_workflow = 4;
     }
 }
 
@@ -80,6 +81,10 @@ message StartWorkflowTaskAttributes {
     //   will be others - workflow exe started attrs, etc
 }
 
+message CancelWorkflowTaskAttributes {
+    // TODO: add attributes here
+}
+
 message TimerFiredTaskAttributes {
     string timer_id = 1;
 }