[kotlin][client] add support for coroutines with OkHttp

[4brunu]

This PR adds support for coroutines with OkHttp http client.

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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)

[wing328]

I did some tests locally and found the following errors:

Starting a Gradle Daemon (subsequent builds will be faster)

> Task :compileKotlin
e: /Users/williamcheng/Code/openapi-generator/samples/openapi3/client/petstore/kotlin-jvm-okhttp4-coroutines/src/main/kotlin/org/openapitools/client/apis/FakeApi.kt: (504, 183): Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type Array<String>?
e: /Users/williamcheng/Code/openapi-generator/samples/openapi3/client/petstore/kotlin-jvm-okhttp4-coroutines/src/main/kotlin/org/openapitools/client/apis/FakeApi.kt: (504, 209): Unresolved reference: collectionDelimiter
e: /Users/williamcheng/Code/openapi-generator/samples/openapi3/client/petstore/kotlin-jvm-okhttp4-coroutines/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt: (136, 21): Type inference failed: Cannot infer type parameter V in inline operator fun <K, V> MutableMap<K, V>.set(key: K, value: V): Unit
None of the following substitutions
receiver: MutableMap<String, List<String>>  arguments: (String,List<String>)
receiver: MutableMap<String, String>  arguments: (String,String)
can be applied to
receiver: MutableMap<String, List<String>>  arguments: (String,String)

e: /Users/williamcheng/Code/openapi-generator/samples/openapi3/client/petstore/kotlin-jvm-okhttp4-coroutines/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt: (138, 21): Type inference failed: Cannot infer type parameter V in inline operator fun <K, V> MutableMap<K, V>.set(key: K, value: V): Unit
None of the following substitutions
receiver: MutableMap<String, List<String>>  arguments: (String,List<String>)
receiver: MutableMap<String, String>  arguments: (String,String)
can be applied to
receiver: MutableMap<String, List<String>>  arguments: (String,String)

Does it work for you locally in your machine?

[4brunu]

You are right, thanks for pointing this!
I submitted a commit and now it's working.
One thing that we could improve in the kotlin generator is to let the CI build the kotlin samples in each PR, like the Swift samples do at the moment.
What do you think on this?

[wing328]

One thing that we could improve in the kotlin generator is to let the CI build the kotlin samples in each PR, like the Swift samples do at the moment.

I prefer to consolidate samples if possible.

Swift also has too many test spec that can be consolidated into 1 or 2. I'll PM later on this.

[4brunu]

Ok 👍
Could we address those issues in different PRs, to unlock this one?

[wing328]

Yes, let's do that in a separate PR.

Let me test this again and merge if no issue found.