MJCF to SDFormat: Support defaults for assets (meshes and materials)

[azeey]

🎉 New feature

Summary

Currently, we only support defaults for items under <worldbody>. This adds support for applying defaults to <mesh> and <material> which are under <assets>. This is needed to convert a non-public model that uses defaults for <mesh>.

Test it

test_defaults.py

Checklist

• [x] Signed all commits for DCO
• [x] 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)
• [x] codecheck passed (See contributing)
• [x] 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[bot]]

Codecov Report

Merging #98 (87380ac) into main (578f6e0) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
+ Coverage   95.14%   95.15%   +0.01%     
==========================================
  Files          24       24              
  Lines        1173     1177       +4     
==========================================
+ Hits         1116     1120       +4     
  Misses         57       57              

Impacted Files	Coverage Δ
...ormat_mjcf/mjcf_to_sdformat/converters/material.py	95.65% <100.00%> (ø)
...sdformat_mjcf/mjcf_to_sdformat/converters/world.py	97.56% <100.00%> (+0.08%)	:arrow_up:
sdformat_mjcf/src/sdformat_mjcf/utils/defaults.py	82.05% <100.00%> (ø)

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