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kotlinlang

Hi guys, would it be possible to bring the `classDiscriminator` from context level to base class level? For example `@Polymorphic(discriminator="command")` or as part of the SerialModule: `polymorphic("command", Message::class) { ... }`  or similar? It would add a great flexibility and it doesn't seem very difficult to add to the current design.

Issue required for start :slightly_smiling_face: <https://github.com/Kotlin/kotlinx.serialization/issues>