How to get force feedback value in bilateral mode

[openarm_bilateral_control_ros.cpp](https://github.com/user-attachments/files/24953373/openarm_bilateral_control_ros.cpp)

### Describe the feature you want

Hi, I’m trying to reimplement bilateral mode in ROS 2, but I couldn’t find any official documentation about the returned force message. Is there an official document that explains the force message format or semantics?