[Kotlin] Fix (de)serialization of enum classes (kotlinx serialization)

Created by: kvn-stgl

Enum class which are inside the dataClass.mustache file are missing the Serializable annotation. This leads to a wrong behavior of the serializer where the names instead of the the SerialName values are used.

You can reproduce this issue if you run the kotlin-retrofit2-kotlinx_serialization sample project. A warning like Explicit @Serializable annotation on enum class is required when @SerialName or @SerialInfo annotations are used on its members. will be thrown.

The changeset also adds the Xopt-in compiler flags for the ExperimentalSerializationApi which also leads to compiler warnings.

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