Use structpb.Value as internal storage of Property

[jhrozek]

Summary

There are several reasons to use structpb.Value over any:

• NewValue contains pretty nice code that can e.g. serialize a []byte slice base64 encoded
• Returning the value as any actually does checks, for example for over/underflow for numerical values and returns a NaN in that case
• there is another draft PR that proposes to use structpb.Struct in the general entity protobuf message which would make the value easily usable should we need to

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

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: 53.724% (-0.04%) from 53.764%
when pulling b4b8bef on jhrozek:prop_proto_struct
into d32c80c on stacklok:main.