Refactor StatusList2021 to use String for credential index

[itsyaasir]

Description of change

This PR refactors the StatusList2021 to use String for the credential index instead of usize. The internal index remains a usize to the compatibility with existing internal logic. Since we know the type is always an integer, we can parse the string to usize and use in the internal logic.

If required, I can also change the internal types to String instead of usize

Links to any relevant issues

Fixes the wrong index type in the source code; Link here

Type of change

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• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

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Change checklist

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• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[UMR1352]

Hold up. The only issue we have with the spec is the fact that our structures serialize to an object having a field with the wrong type but the overall interface is fine. I would rather change the way the structures are serialized (using serde_aux or a custom serialization function perhaps).

[itsyaasir]

Redoing it in another branch