Add a way to make it required to follow a ValueProvider

[KennethHoff]

Currently, [ValueProviderAttribute] has a single parameter; The source. Would you be willing to add another parameter (Or simply a property) that makes it so you have to choose a value from that Provider? I would love to say "You have to choose a value from this", and not simply a suggestion.

[Klikini]

I would also like this, especially since it would presumably only show values from the given provider in the autocomplete menu instead of just listing them first.