Read the webhook secret from a file

[jhrozek]

Summary

Complements reading the previous webhook secret from a file by allowing
to read the current one.

Fixes: #3160

Change Type

• 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

make test, use the option

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: 50.138% (+0.02%) from 50.119%
when pulling a8710e1 on jhrozek:wh_from_file
into cea2520 on stacklok:main.