Add support for Kotlin nullable types

[mschorsch]

See https://github.com/quarkusio/quarkus/issues/27319

[radcortez]

I looked into this, and supporting it may require some work. Unfortunately, there doesn't seem to exist a way to determine if a type is nullable or not on the Java side https://youtrack.jetbrains.com/issue/KT-14909.

Either we need to create a separate module to access the Kotlin API and query Kotlin reflection, or we need to provide an annotation processor and look for the nullable annotations. On top of this, the mapping API will require some changes to allow consuming external metadata and merging it with the metadata read from Java.

It would be great if someone helps with this. Or maybe even have a better idea of how to support it.