[kotlin-client] Support for integer enums for multiplatform with custom serializer

[rlnt]

Extends #21765 to support the multiplatform library.
Fixes #21204

I didn't add a separate serializer for the handling of enumUnknownDefaultCase, since this was unhandled previously as well.

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[wing328]

cc @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03) @stefankoppier (2022/06) @e5l (2024/10)

[wing328]

closed via #23211 with tests to cover the change