Add contract test for UserIdentifier

[ruspl-afed]

Extracted from #1454 (review) by @eparovyshnaya

UserIdentifier interface is used by core, among other ways, as a key to HashMaps.
This usage lies obligations on this interface (hashCode() and equals() impls should be in synch).

The only implementation of this interface is record, so it meets the requirement automatically. But custom implementations of Access Cycle Configuration services can potentially supply their own implementation for UserIdentifier interface as well.

This means that there should be a contract test for this interface.

We rely on UserIdentifier implementation to

• do not tolerate null as construction input
• have synchronous equals() and hashCode(), meaning different instances, constructed with equal strings, beget true for equals() and have the same hashCode().

This contract test must be implemented for BaseUserIdentifier.