dartsim: Add support for joints in worlds

[azeey]

🎉 New feature

Closes #457

Summary

As of gazebosim/sdformat#1117, SDFormat supports joints under world (//world/joint in xpath). But in gz-physics, Entities such as joints are only available in a Model. To support world joints, instead of creating a new type of entity, we have chosen to implement a scheme where the world behaves like a model via the WorldModelFeature feature. Given a world, w, we can do model = w->GetWorldModel() to get the model proxy. We can then use any of the APIs supported by the features available in the physics engine, e.g., model->GetJoint("j1") where "j1" is a joint directly under <world> in the SDFormat.

Some refactoring of Base.hh has already been split out into #500, so that should be merged first.

Test it

Tests have been added to tests test/common_test/joint_features.cc and test/common_test/world_features.cc

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.

[codecov]

Codecov Report

Merging #501 (0926e48) into gz-physics6 (63a7486) will increase coverage by 0.44%.
The diff coverage is 77.61%.

❗ Current head 0926e48 differs from pull request most recent head 6b70010. Consider uploading reports for the commit 6b70010 to get more accurate results

@@               Coverage Diff               @@
##           gz-physics6     #501      +/-   ##
===============================================
+ Coverage        75.71%   76.16%   +0.44%     
===============================================
  Files              142      142              
  Lines             7199     7251      +52     
===============================================
+ Hits              5451     5523      +72     
+ Misses            1748     1728      -20     

Impacted Files	Coverage Δ
dartsim/src/SDFFeatures.cc	64.71% <60.34%> (+2.05%)	⬆️
dartsim/src/Base.hh	95.85% <89.74%> (+0.85%)	⬆️
dartsim/src/EntityManagementFeatures.cc	86.52% <90.90%> (+3.45%)	⬆️
include/gz/physics/detail/GetEntities.hh	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes