Refactor sdf::readXml

[azeey]

Summary

While working on a new feature for the main branch I got a codecheck error saying sdf::readXml is over 500 lines. This simply refactors two sections of the the function into helper functions.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[aaronchongth]

Thanks for refactoring readXml! I think we were just hitting the limit of allowed lines in a single function too 😆, so this is going to make future parsing work much easier. I have a few trivial comments, but everything looks great!