Publish Joint forces

[Fixit-Davide]

🦟 Bug fix

Fixes #883

Summary

These changes should allow to obtain information regarding the forces applied to the joints.

Test it

• Pull gz-physics PR: https://github.com/gazebosim/gz-physics/pull/628
• Launch gz sim --physics-engine gz-physics-bullet-featherstone-plugin lift_drag.sdf simulation.
• Send a command to the model (gz topic -t "/model/lift_drag_demo_model/joint/rod_1_joint/cmd_force" -m gz.msgs.Double -p "data: 0.8").
• Echo the topic: gz topic -e -t /world/lift_drag/model/lift_drag_demo_model/joint_state

  Checklist

• [x] 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.

[azeey]

Also, can you please add a test?

[Fixit-Davide]

I have just made a PR in gz-physics in order to populate the force fields while using bullet-featherstone. I added some information on how to test the code there (I have reported them here also).