* [kotlin] Make API classes open (non-final) unless nonPublicApi is used
By making those classes open, AspectJ can be used to enhance their
behavior.
I'm not changing kotlin-multiplatform templates, where 'open' was
already present before my changes.
Closes#22271
* Fail Kotlin sample compilation if API classes stop to be public
Serves as a 'regression test'.
* [kotlin-client] Add support for integer enums with custom serializer for kotlinx
* [kotlin-client] Fix @SerialName annotation being used on non-string enums
* Format samples
* remove blank line
* add tests for kotlin client with integer enum
* add folders
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Co-authored-by: CHervaudBetclic <c.hervaud@betclicgroup.com>