Refactor: Move SetViperStructDefaults to utils

[Vyom-Yadav]

Summary

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This change was reverted in #2822. #2839 was merged to solve the config overriding problem.

Change Type

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• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

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Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 47.36% (+0.01%) from 47.348%
when pulling 2fa1d82 on Vyom-Yadav:mv-to-utils
into 6f69727 on stacklok:main.

[Vyom-Yadav]

@jhrozek @JAORMX If you're around, can we merge this? Just refactoring so the function can be used at #2638