Add support for generating a Set for a list with uniqueItems set to true

Created by: marcdejonge

To fix #9848 (closed)

This specifically adds support for a Set type when a model with an array has uniqueItems set to true. This only changes the KotlinClientGenerator template, since that one was missing specific support for it. The tests run agains all 5 generators, but the rest seem to work fine.

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