"Enum unspecified value" rule inaccurate to AIP

[mbleigh]

Steps to reproduce

1. Write an enum with a meaningful zero value
2. Enable strict API linter
3. Linter flags missing _UNSPECIFIED value

The issue here is that the AIP itself has an exception for a "clearly useful zero value". This means that even in the strict API linter it should be possible to have an enum with a meaningful zero value because it's an exception to the rule, not just an ignored should.