[Bug] Make UserAssignedManagedIdentityClientId an instance field on a new interface, not settable externally

[jennyf19]

Having this be static and externally settable opens up race conditions where someone thinks it is populated when it may not be, and then makes incorrect decisions based upon that. Saying it is not used that way now is not sufficient. This is a high risk of misuse for future contributors. Recommendation is to make it an instance field on a new interface that is not settable externally, then access it through dependency injection. -> from