Add -Wconversion flag to protect future developments (backport #667)

[mergify]

Same reasons as ros-controls/ros2_control#1053

explicitly catching errors of time being zero in forward_command_controller see
I think there might be an ABI breaking in JTC with one argument changed from int to size_t. I did not know what to do there, and went for ABI breaking but I could also make it a static_cast if preferred. see

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This is an automatic backport of pull request #667 done by [Mergify](https://mergify.com).

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 65.16%. Comparing base (da5c15d) to head (e20295e).
Report is 1 commits behind head on humble.

Files with missing lines	Patch %	Lines
tricycle_controller/src/tricycle_controller.cpp	60.00%	2 Missing ⚠️
tricycle_controller/src/traction_limiter.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           humble    #1321      +/-   ##
==========================================
- Coverage   65.23%   65.16%   -0.08%     
==========================================
  Files         103      103              
  Lines       11437    11995     +558     
  Branches     7254     7398     +144     
==========================================
+ Hits         7461     7816     +355     
- Misses       1483     1544      +61     
- Partials     2493     2635     +142     

Flag	Coverage Δ
unittests	65.16% <62.50%> (-0.08%)	⬇️

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

Files with missing lines	Coverage Δ
...iff_drive_controller/src/diff_drive_controller.cpp	73.06% <100.00%> (+0.84%)	⬆️
...ontroller/test/test_forward_command_controller.cpp	57.89% <ø> (-1.39%)	⬇️
...est_multi_interface_forward_command_controller.cpp	62.50% <ø> (-1.91%)	⬇️
...include/joint_trajectory_controller/tolerances.hpp	80.86% <ø> (+0.68%)	⬆️
...ory_controller/src/joint_trajectory_controller.cpp	80.72% <ø> (+0.10%)	⬆️
tricycle_controller/src/traction_limiter.cpp	22.22% <0.00%> (+1.25%)	⬆️
tricycle_controller/src/tricycle_controller.cpp	66.18% <60.00%> (-0.79%)	⬇️

... and 77 files with indirect coverage changes