Kotlin: Improve KotlinSerializerCodec

[rozza]

Only support classes annotated as serializable.

JAVA-5035

[rozza]

This way we still support any explicitly declared classes that are Serializable.

The previous check for kClass.isSealed also picked up Java Sealed interfaces and would error.

Note: Java records didn't return true from the previous check (kClass.isData || kClass.isValue)

[rozza]

Other approaches check for the presence of the kotlin Metadata annotation. However, we only want @Serializable kotlin classes.