Fallback Deserializer for unknown types

[findusl]

If the commands are extended in the future, but the client is not updated, the client might receive a command it cannot recognise. For this purpose the Serialisers should be extended to allow for a fallback "UnknownCommand" value to be used.

Kotlin/kotlinx.serialization#448

Unclear how to make this clean, except by making a separate FallbackSerialiser for each object. Maybe it would be possible to add a different annotation that defines the fallback and read that. Need to check about annotation retention in KMM