Function ValidateByAsValidatorFunction

This is the drop-in replacement for class-validator's ValidateBy() function.

The compiled version needs to be injected into class-validator, (i.e exports.ValidateBy = ValidateByAsValidatorFunction) to be able to use any (?) PropertyDecorator as a function!

wrapValidateByPropertyDecorator for for context!

@todo(4 5 7): handle async/promises @todo(5 6 8): explain better