Make generated classes support `Serializable` under Kotlin/JVM

[garyp]

Classes generated by pbandk should implement Serializable under Kotlin/JVM so that they can be included inside of other Serializable classes. See #140 for some motivating use cases. This would provide feature parity with protobuf-java, which does generate classes that implement Serializable.

This is hard to implement directly in the generated code since the generated code can only reference classes available to the common sourceset of a Kotlin Multiplatform project, but the java.io.Serializable interface is only available under Kotlin/JVM. However, #31 is going to change the classes generated by pbandk to extend from a pbandk.GeneratedMessage class. Once that is in place, we can use expect/actual to create different implementations of GeneratedMessage for each Kotlin platform. Then the Kotlin/JVM implementation of GeneratedMessage can implement java.io.Serializable and custom readObject()/writeObject() methods as needed (or maybe we'll need to use writeReplace() and readResolve()?).