Actuators message for JointPositionController.

[bperseghetti]

🎉 New feature

Adds ability to control the position of joints with the position field of the actuators message.

Closes: #1974

As seen in the Gazebo community meeting:
https://vimeo.com/812911652#t=39m58s

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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.

[bperseghetti]

Still needs the tests added.

[codecov]

Codecov Report

Merging #1954 (ad1d504) into gz-sim7 (859cd93) will increase coverage by 0.36%.
The diff coverage is 87.45%.

❗ Current head ad1d504 differs from pull request most recent head b7671af. Consider uploading reports for the commit b7671af to get more accurate results

@@             Coverage Diff             @@
##           gz-sim7    #1954      +/-   ##
===========================================
+ Coverage    64.62%   64.98%   +0.36%     
===========================================
  Files          348      352       +4     
  Lines        27859    28395     +536     
===========================================
+ Hits         18004    18453     +449     
- Misses        9855     9942      +87     

Impacted Files	Coverage Δ
src/SimulationRunner.hh	100.00% <ø> (ø)
...rc/systems/ackermann_steering/AckermannSteering.hh	100.00% <ø> (ø)
...int_position_controller/JointPositionController.hh	100.00% <ø> (ø)
include/gz/sim/EventManager.hh	75.75% <66.66%> (-3.41%)	⬇️
...int_position_controller/JointPositionController.cc	73.17% <69.69%> (-0.97%)	⬇️
src/systems/sensors/Sensors.cc	62.64% <70.58%> (-0.37%)	⬇️
src/rendering/RenderUtil.cc	38.71% <72.22%> (-0.83%)	⬇️
...rc/systems/ackermann_steering/AckermannSteering.cc	80.09% <73.52%> (-0.45%)	⬇️
src/Sensor.cc	85.71% <85.71%> (ø)
src/Light.cc	93.11% <93.11%> (ø)
... and 3 more

... and 2 files with indirect coverage changes

[bperseghetti]

@azeey @mjcarroll good to merge now?

[bperseghetti]

@mjcarroll I updated what you requested.

I also force pushed the same changes to JC PR #1953

As well as fixed Ackermann and unified MulticopterMotorModel with a deprecation warning and ability to use same <actuator_number>.