Refactor: Trajectory loading seperated into external function

[onurtore]

🦟 Refactor

Summary

Refactors the trajectory loading code snippet into a separate function.

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.

[codecov]

Codecov Report

Merging #1744 (9c37be8) into gz-sim7 (74854fd) will increase coverage by 0.00%.
The diff coverage is 68.75%.

@@           Coverage Diff            @@
##           gz-sim7    #1744   +/-   ##
========================================
  Coverage    63.90%   63.90%           
========================================
  Files          334      334           
  Lines        26338    26340    +2     
========================================
+ Hits         16830    16833    +3     
+ Misses        9508     9507    -1     

Impacted Files	Coverage Δ
src/rendering/SceneManager.cc	28.54% <68.75%> (+0.20%)	⬆️

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[onurtore]

Ubuntu CI failed because it could not find the test file, however, this PR does not remove any test files, windows failed because it couldnt find some files which are not related to this PR.

[mjcarroll]

@osrf-jenkins retest this please

[mjcarroll]

CI failures are unrelated to this PR.