[Steering controllers library] Reference interfaces are body twist (backport #1168)

[mergify[bot]]

Fixes #1167

I'm not sure if the intention of the author was to have position input, but as it is implemented we have a body twist input on reference interfaces (velocity, not position). This PR fixes this, without any attempt to deprecate the old behavior because it was just wrong.

Tests succeed on iron distro

$ colcon test --packages-select steering_controllers_library ackermann_steering_controller bicycle_steering_controller tricycl
e_steering_controller
Starting >>> steering_controllers_library
Finished <<< steering_controllers_library [0.48s]          
Starting >>> ackermann_steering_controller
Starting >>> bicycle_steering_controller
Starting >>> tricycle_steering_controller
Finished <<< tricycle_steering_controller [7.25s]                                                                                                  
Finished <<< ackermann_steering_controller [7.28s]
Finished <<< bicycle_steering_controller [7.33s]

Summary: 4 packages finished [8.33s]
```<hr>This is an automatic backport of pull request #1168 done by [Mergify](https://mergify.com).