Add lint rule against fixed GCP URLs appearing in code

[rileykarson]

What kind of contribution is this issue about?

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Details

Fixed GCP URLs like ""https://servicedirectory.googleapis.com/v1/"" are unsafe to use as universe domain and custom endpoint substitution will no longer function correctly. We need to check BasePaths like ServiceDirectoryBasePath. Misses were par for the course when custom endpoints were a mostly hacky solution but we want to preempt these kind of issues more for custom universes now.

We should lint against new fixed GCP URLs appearing outside of "safe" locations like the definition of GCP default URLs for a service.

References

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