One of support for lagom

Created by: fokot

Hi @clasnake (2017/07), @jimschubert (2017/09) , @shijinkui (2018/01), @ramzimaalej (2018/03), @chameleon82 (2020/03), @Bouillie (2020/04)

• This adds oneOf support for Lagom. I can not use sealed trait and Format derivation as generator generates every class to separate file.
• And it support only one way of writing this in openapi - that is you need to have type which as oneOf attribute not just a property.
• Files in /generated folder are what will be generated from oneoOfDiscriminatorSimple.yaml. I will remove them they are here so just you can tell what you think bout that without running it.

What do you think? Merging it would help our team. Thanks

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