Allow reading secrets from file for GH App config

[eleftherias]

Summary

In our deployment we need to read the fallback token and webhook secret from files.

Fixes #3039
Fixes #3038

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: 48.216% (-0.004%) from 48.22%
when pulling 15d754b on eleftherias:gh-config-from-file
into 471f6de on stacklok:main.