Decide the output of BtServiceNode based on the service-response (#1615)

- `BtServiceNode::check_future()` was created, to encapsulate the logic
where a the output of the BtServiceNode is computed.
- Inherited classes can overload this function according to the requirement
of the user

nav2_behavior_tree/include/nav2_behavior_tree/bt_service_node.hpp

@@ -85,7 +85,19 @@ class BtServiceNode : public BT::SyncActionNode
   {
     on_tick();
     auto future_result = service_client_->async_send_request(request_);
+    return check_future(future_result);
+  }
+
+  // Fill in service request with information if necessary
+  virtual void on_tick()
+  {
+    request_ = std::make_shared<typename ServiceT::Request>();
+  }
 
+  // Check the future and decide the status of Behaviortree
+  virtual BT::NodeStatus check_future(
+    std::shared_future<typename ServiceT::Response::SharedPtr> future_result)
+  {
     rclcpp::executor::FutureReturnCode rc;
     rc = rclcpp::spin_until_future_complete(
       node_,
@@ -101,12 +113,6 @@ class BtServiceNode : public BT::SyncActionNode
     return BT::NodeStatus::FAILURE;
   }
 
-  // Fill in service request with information if necessary
-  virtual void on_tick()
-  {
-    request_ = std::make_shared<typename ServiceT::Request>();
-  }
-
   // An opportunity to do something after
   // a timeout waiting for a result that hasn't been received yet
   virtual void on_wait_for_result()