Detect primary constructor to generate metadata for Kotlin immutable types

[snicoll]

Configuration keys metadata for immutable Kotlin types are likely not to be generated as a single constructor in Kotlin may lead to several constructor on the Java side.

We should detect the primary constructor, if any and, for that, we need to access the metadata available only in binary format for now.

A couple of libraries could help with that:

• https://github.com/JetBrains/kotlin/blob/master/libraries/kotlinx-metadata/jvm/ReadMe.md
• https://github.com/Takhion/kotlin-metadata

[sdeleuze]

Notice that unlike what is mentioned in the documentation, kotlinx-metadata-jvm is available from Maven Central without requiring addition repository configuration.

As discussed with @snicoll, I think the ideal solution would be a version of kotlinx-metadata-jvm without a dependency on kotlin-stdlib (using Proguard maybe) that we could shade in spring-boot-configuration-processor.