Handle int64 and uint64 in properties by wrapping it

[jhrozek]

Summary

We started using structpb.Value to hold the payload of our properties,
but since the Value type is expected to be directly serialized to a JSON
value, all numerical values are treated as float64.

Because we also want to store int64 values, we need to treat them
separately, by storing them as objects instead with the integer value
stringified.

Related: #4171

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

mostly unit tests

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: 53.838% (+0.1%) from 53.716%
when pulling 1298886 on jhrozek:prop_int64
into b30ca2e on stacklok:main.