Separate jwt functionality into its own package

[eleftherias]

Summary

This moves any JWT specific auth code into its own package.
There are not functional changes.
This is intended to make the code more testable.

Change Type

Mark the type of change your PR introduces:

• 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

Outline how the changes were tested, including steps to reproduce and any relevant configurations.
Attach screenshots if helpful.

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: 51.893% (+0.009%) from 51.884%
when pulling 35f7263 on eleftherias:jwt-separate-package
into 5106a3c on stacklok:main.

[coveralls]

coverage: 51.893% (+0.009%) from 51.884%
when pulling 1f8e1ee on eleftherias:jwt-separate-package
into 5106a3c on stacklok:main.