Align PropertyDescriptor asAllowableValue with Builder Declaration

NIFI-12452 introduced a new method on PropertyValue to type-safely access a property with allowableValues constrained by an Enum. 

<E extends Enum<E> & DescribedValue> E asDescribedValue(Class<E> enumType) throws IllegalArgumentException 

I think it makes sense to align the access site in PropertyValue with the declaration site in PropertyDescriptor.Builder.

This would involve renaming the method to asAllowableValue for improved symmetry.
This is a breaking change, however the method was never part of an stable release.

Additionally, NIFI-12573 unified the behaviour of specifying Enums (not) implementing DescribedValue as allowableValues. With this change in place, I think it's reasonable to open the method to accepts any Enum as well.