Add ref from update completed to accepted (#296)

Helps observers to follow the chain of execution for a single update.

Author: Matt McShane

temporal/api/history/v1/message.proto

@@ -725,6 +725,10 @@ message WorkflowExecutionUpdateAcceptedEventAttributes {
 message WorkflowExecutionUpdateCompletedEventAttributes {
     // The metadata about this update.
     temporal.api.update.v1.Meta meta = 1;
+
+    // The event ID indicating the acceptance of this update.
+    int64 accepted_event_id = 3;
+
     // The outcome of executing the workflow update function.
     temporal.api.update.v1.Outcome outcome = 2;
 }