Support for conditional validation with class-validator

[iamciroja]

No description provided.

[Brakebein]

Looks solid to me. It also adds @ValidateIf in general, which is nice.

It works for simple expressions. Other use cases may include calling a special function inside, e.g. @ValidateIf(o => runCustomTest(o)). However, this needs more complex parsing and collecting imports similar to the @DtoCastType decorator. I think we can keep it simple for now and may extend it when someone requests it.