Single implementation for idempotency cache

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We create IdempotencyCache for the purposes of ensuring the updates to the hosted runner usage are idempotent when the worker retries. !180449 (diffs)

However, there are a few improvements we can make:

1. Shared implementation: We should share the implementation with Ci::Minutes::UpdateProjectAndNamespaceUsageService.
   • I'm not doing it in !180449 (merged) because changes to this area are risky and it should be properly flagged.