"Enum unspecified value" rule inaccurate to AIP

#### 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](https://google.aip.dev/126) 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**.