[Kotlin][Client] new library support: Retrofit 2.x

Created by: javils

#4517

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PR Description

This PR add support for retrofit2 as a HTTP library in kotlin-client generator. The reason to add this feature is that I want to port some old android java code to kotlin, but I couldn't because retrofit2 was not supported.

@jimschubert @dr4ke616 @karismann @Zomzog @andrewemery