dartsim: Refactor entity addition logic in the EntityStorage class

[azeey]

🎉 New feature

Summary

The entity addition logic is for each type of entity. This refactors it to an AddEntity member function. This will be used in a follow up PR for supporting world joints.

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 #500 (dc05a93) into gz-physics6 (8736107) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

❗ Current head dc05a93 differs from pull request most recent head 9044f21. Consider uploading reports for the commit 9044f21 to get more accurate results

@@               Coverage Diff               @@
##           gz-physics6     #500      +/-   ##
===============================================
- Coverage        75.56%   75.50%   -0.07%     
===============================================
  Files              142      142              
  Lines             7191     7173      -18     
===============================================
- Hits              5434     5416      -18     
  Misses            1757     1757              

Impacted Files	Coverage Δ
dartsim/src/Base.hh	95.00% <100.00%> (-0.38%)	⬇️

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[azeey]

@osrf-jenkins run tests please

[azeey]

The failing tests are also failing on the stable branch CI, so I'll go ahead and merge.