Add tests for Kotlin non-null and non-mutable compatibility

[tlinkowski]

The idea is to reflectively query sample-api from Kotlin (like in sample-project-kotlin), then call toString and ensure there are no ! nor Mutable substrings.

Blocked by:

• KT-28009: Support JSR-305 nullability annotations in reflection (could be worked around by using compiler and verifying that there's an error, but it's much more work - example)
• KT-28992: Type inference problem in Kotlin when calling Java code annotated with @ReadOnly
• KT-28982: Add ElementType.TYPE_USE to @Target of @ReadOnly and @Mutable annotations

[tlinkowski]

Duplicates #9.