NIFI-12613 Align type-safe access to allowableValue with its declaration

[EndzeitBegins]

Summary

NIFI-12613

Aligns the access site of "allowableValues" in PropertyValue with the declaration site in PropertyDescriptor.Builder. The method asDescribedValue was introduced recently and hasn't been part of a proper release yet. I think its a good time to align the type and naming scheme with the configuration site before the inclusion in a first release, as it would be a breaking change then.

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[exceptionfactory]

Thanks for introducing this change @EndzeitBegins, the approach looks good, and as you noted, this is a good time to make the change prior to the next milestone release. +1 merging

[EndzeitBegins]

Thank you for the quick review & merge @exceptionfactory. 🙂