Actuators message input for JointController.

[bperseghetti]

🎉 New feature

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

Closes: #1973

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 need to add tests.

[codecov]

Codecov Report

Merging #1953 (42b3aea) into gz-sim7 (859cd93) will increase coverage by 0.40%.
The diff coverage is 85.12%.

❗ Current head 42b3aea differs from pull request most recent head f049217. Consider uploading reports for the commit f049217 to get more accurate results

@@             Coverage Diff             @@
##           gz-sim7    #1953      +/-   ##
===========================================
+ Coverage    64.62%   65.03%   +0.40%     
===========================================
  Files          348      352       +4     
  Lines        27859    28468     +609     
===========================================
+ Hits         18004    18514     +510     
- Misses        9855     9954      +99     

Impacted Files	Coverage Δ
include/gz/sim/rendering/MarkerManager.hh	100.00% <ø> (ø)
include/gz/sim/rendering/RenderUtil.hh	100.00% <ø> (ø)
include/gz/sim/rendering/SceneManager.hh	100.00% <ø> (ø)
src/SimulationRunner.hh	100.00% <ø> (ø)
...rc/systems/ackermann_steering/AckermannSteering.hh	100.00% <ø> (ø)
src/systems/joint_controller/JointController.hh	100.00% <ø> (ø)
...int_position_controller/JointPositionController.hh	100.00% <ø> (ø)
src/systems/physics/Physics.cc	67.13% <0.00%> (-0.09%)	⬇️
src/SdfEntityCreator.cc	86.49% <25.00%> (-0.53%)	⬇️
include/gz/sim/EventManager.hh	75.75% <66.66%> (-3.41%)	⬇️
... and 13 more

... and 2 files with indirect coverage changes

[bperseghetti]

@azeey @mjcarroll good to merge now?

[mjcarroll]

@osrf-jenkins retest this please