Add Model::CanonicalLink getter

[chapulina]

🎉 New feature

• Broken off New Apply Link Wrench system #1593

Summary

Add a convenient function to get a model's canonical link.

Test it

See the added tests.

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 #1594 (b79126b) into ign-gazebo3 (df45262) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           ign-gazebo3    #1594      +/-   ##
===============================================
- Coverage        77.76%   77.72%   -0.04%     
===============================================
  Files              250      250              
  Lines            14443    14447       +4     
===============================================
- Hits             11231    11229       -2     
- Misses            3212     3218       +6     

Impacted Files	Coverage Δ
include/ignition/gazebo/Model.hh	100.00% <ø> (ø)
src/Model.cc	95.83% <100.00%> (+0.24%)	⬆️
src/SimulationRunner.cc	93.45% <0.00%> (-1.07%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update df45262...b79126b. Read the comment docs.