Add P gain value for Ackermann steering.

[bperseghetti]

🦟 Bug fix

Fixes #1872

Summary

Adds P gain for ackermann steering when:

 <steering_only>true</steering_only>

Set with:

<steer_p_gain>your_value</steer_p_gain>

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #1873 (ba980bc) into gz-sim7 (af73ebe) will increase coverage by 0.02%.
The diff coverage is 50.00%.

@@             Coverage Diff             @@
##           gz-sim7    #1873      +/-   ##
===========================================
+ Coverage    64.64%   64.67%   +0.02%     
===========================================
  Files          343      343              
  Lines        27542    27544       +2     
===========================================
+ Hits         17805    17813       +8     
+ Misses        9737     9731       -6     

Impacted Files	Coverage Δ
...rc/systems/ackermann_steering/AckermannSteering.hh	100.00% <ø> (ø)
...rc/systems/ackermann_steering/AckermannSteering.cc	80.54% <50.00%> (+0.09%)	⬆️
src/SimulationRunner.cc	91.20% <0.00%> (+0.94%)	⬆️

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