Currently `ValidateSetAttribute` accepts only explicit constants as valid values. This is a significant limitation. Sometimes we need to get valid values dynamically, ex., Azure VMs, logged-on users and so on. The PR add follow possibilities:
- pass a _custom type_ (a valid values generator) implementing `IValidateSetValuesGenerator` interface to get valid values dynamically.
- pass a _custom type_ (a valid values generator) derived from `CachedValidValuesGeneratorBase` abstract class to get valid values dynamically and _cache_ the list to share with other ValidateSetAttribute attributes.