Add joint force/torque getter

[jslee02]

[Remove this line and describe this pull request. Link to relevant GitHub issues, if any.]

---

Before creating a pull request

• Document new methods and classes
• Format new code files using clang-format

Before merging a pull request

• Set version target by selecting a milestone on the right side
• Summarize this change in CHANGELOG.md
• Add unit test(s) for this change

[codecov]

Codecov Report

Merging #1616 (6e5488f) into main (0cc469b) will increase coverage by 0.01%.
The diff coverage is 73.33%.

❗ Current head 6e5488f differs from pull request most recent head 70cd90a. Consider uploading reports for the commit 70cd90a to get more accurate results

@@            Coverage Diff             @@
##             main    #1616      +/-   ##
==========================================
+ Coverage   59.06%   59.08%   +0.01%     
==========================================
  Files         477      477              
  Lines       39880    39895      +15     
==========================================
+ Hits        23556    23572      +16     
+ Misses      16324    16323       -1     

Impacted Files	Coverage Δ
dart/dynamics/Joint.hpp	100.00% <ø> (ø)
dart/dynamics/Joint.cpp	59.03% <73.33%> (+3.05%)	⬆️