[REQ] [Kotlin] Use kotlin.Set for an array with uniqueItems set to true

Created by: marcdejonge

When defining an array with uniqueItems set to true, I expect that the type is a Set. But right now it's always a List.

Describe the solution you'd like

Add support for the Set type.

Describe alternatives you've considered

Right now I'm using I'm using custom templates to generate this. I think this would be useful to be part of the default templates.