[Kotlin] Introduce support for Kotlin Explicit API Mode for Kotlin-Client generator (resolve #16506)

[xsveda]

Resolves #16506

Kotlin Explicit API Mode requires all public APIs to be explicitly marked as public and have explicit return type declared.

This PR adds an explicitApi option to Kotlin-Client codegen to cover this requirement.

A kotlin-explicit sample has been added as a test case but the same behaviour has been tested also for Kotlin Multiplatform & Kotlin Retrofit.

@dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03) @stefankoppier (2022/06) @e5l (2024/10)

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[4brunu]

Thanks, this is a great contribution, looks good to me 👍

[martintimko-kosik]

Hi @xsveda, did you consider adding the same, for kotlin-spring generator too? would be great 🙏