IUserTaskEvent
oneOf- UserTaskExecutedEvent
 - UserTaskAssignedEvent
 - UserTaskCancelledEvent
 
wfRunIdstringrequired
userTaskGuidstringrequired
oldUserIdstring
oldUserGroupstring
newUserIdstring
newUserGroupstring
messagestringrequired
IUserTaskEvent
{
  "wfRunId": "string",
  "userTaskGuid": "string"
}