Support multiple collisions per link in bullet-featherstone

[nkoenig]

🦟 Bug fix

Only the first collision element in a link was created when using bullet-featherstone. This PR adds each collision shape as a childShape to the bullet collider.

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)
• 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.

[mjcarroll]

LGTM with green CI

[codecov]

Codecov Report

Merging #505 (5384d1b) into gz-physics6 (35d02fe) will increase coverage by 0.00%.
The diff coverage is 53.84%.

❗ Current head 5384d1b differs from pull request most recent head a34d068. Consider uploading reports for the commit a34d068 to get more accurate results

@@             Coverage Diff              @@
##           gz-physics6     #505   +/-   ##
============================================
  Coverage        75.50%   75.51%           
============================================
  Files              142      142           
  Lines             7173     7175    +2     
============================================
+ Hits              5416     5418    +2     
  Misses            1757     1757           

Impacted Files	Coverage Δ
bullet-featherstone/src/SDFFeatures.cc	71.97% <53.84%> (+0.11%)	⬆️

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