[Kotlin] Make ApiClient in jvm-retrofit2 be able to add additional retrofit Converter.Factory

[shanselm-ergon]

[Kotlin][#7925] Make ApiClient in jvm-retrofit2 be able to add additional retrofit Converter.Factory

The serializer option is not enough, we need an additional Converter.Factory for date serialization in query params (not body).

fixes #7925

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@jimschubert (2017/09) ❤️, @dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03)

[4brunu]

Looks good to me 👍

[wing328]

Tested locally and the result is good;

> Task :compileKotlin
w: /Users/williamcheng/Code/openapi-generator/samples/client/petstore/kotlin-retrofit2/src/main/kotlin/org/openapitools/client/auth/OAuthOkHttpClient.kt: (42, 72): 'create(MediaType?, String): RequestBody' is deprecated. Moved to extension function. Put the 'content' argument first to fix Java

Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.8.3/userguide/command_line_interface.html#sec:command_line_warnings

BUILD SUCCESSFUL in 1m 1s
1 actionable task: 1 executed

[wing328]

Travis CI failure not related to this change.