Fix const-correctness of the Model::JointByName and Model::LinkByName APIs

[arjo129]

🦟 Bug fix

Fixes #

Summary

This has been a longstanding pet-peeve of mine and a todo in the code base. Unfortunately, this change likely breaks ABI and hence can only be done in a major release. It does not however break API and hence old code should not have any issues compiling against this new change.

An alternative would be to introduce these const functions as duplicates in citadel and deprecate the non-const functions in garden, but this feels like more work.

Checklist

• 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)
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[codecov]

Codecov Report

Merging #2059 (5df2724) into main (dbebc39) will increase coverage by 0.00%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

❗ Current head 5df2724 differs from pull request most recent head 4d8c788. Consider uploading reports for the commit 4d8c788 to get more accurate results

@@           Coverage Diff           @@
##             main    #2059   +/-   ##
=======================================
  Coverage   65.45%   65.45%           
=======================================
  Files         312      312           
  Lines       29808    29810    +2     
=======================================
+ Hits        19510    19512    +2     
  Misses      10298    10298           

Files Changed	Coverage Δ
include/gz/sim/components/Component.hh	100.00% <ø> (ø)
src/Model.cc	96.15% <ø> (ø)
include/gz/sim/components/Factory.hh	97.29% <100.00%> (+0.02%)	⬆️
src/ComponentFactory.cc	100.00% <100.00%> (ø)