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

[mergify]

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).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.43%. Comparing base (014848c) to head (03ba099).

Additional details and impacted files

@@            Coverage Diff             @@
##             iron    #1174      +/-   ##
==========================================
- Coverage   86.76%   86.43%   -0.33%     
==========================================
  Files          91       91              
  Lines        7873     7874       +1     
  Branches      688      689       +1     
==========================================
- Hits         6831     6806      -25     
- Misses        795      819      +24     
- Partials      247      249       +2     

Flag	Coverage Δ
unittests	86.43% <100.00%> (-0.33%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...roller/test/test_ackermann_steering_controller.cpp	100.00% <ø> (ø)
...roller/test/test_ackermann_steering_controller.hpp	84.21% <ø> (ø)
...ntroller/test/test_bicycle_steering_controller.cpp	100.00% <ø> (ø)
...ntroller/test/test_bicycle_steering_controller.hpp	80.00% <ø> (ø)
...llers_library/src/steering_controllers_library.cpp	74.16% <100.00%> (ø)
...library/test/test_steering_controllers_library.cpp	100.00% <ø> (ø)
...library/test/test_steering_controllers_library.hpp	95.52% <ø> (ø)
...troller/test/test_tricycle_steering_controller.hpp	82.35% <ø> (ø)

... and 6 files with indirect coverage changes