[Kotlin] Make ApiClient in jvm-retrofit2 be able to use own OkHttpClient (#6999)

* added okHttpClient as parameter to the constructor, adapted createService

* updated sample

Author: tgerth

modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-retrofit2/infrastructure/ApiClient.kt.mustache

@@ -48,7 +48,8 @@ import retrofit2.converter.moshi.MoshiConverterFactory
 {{#nonPublicApi}}internal {{/nonPublicApi}}class ApiClient(
     private var baseUrl: String = defaultBasePath,
     private val okHttpClientBuilder: OkHttpClient.Builder? = null,
-    private val serializerBuilder: {{#gson}}Gson{{/gson}}{{#moshi}}Moshi.{{/moshi}}Builder = Serializer.{{#gson}}gson{{/gson}}{{#moshi}}moshi{{/moshi}}Builder
+    private val serializerBuilder: {{#gson}}Gson{{/gson}}{{#moshi}}Moshi.{{/moshi}}Builder = Serializer.{{#gson}}gson{{/gson}}{{#moshi}}moshi{{/moshi}}Builder,
+    private val okHttpClient : OkHttpClient? = null
 ) {
     private val apiAuthorizations = mutableMapOf<String, Interceptor>()
     var logger: ((String) -> Unit)? = null
@@ -286,7 +287,9 @@ import retrofit2.converter.moshi.MoshiConverterFactory
     }
 
     fun <S> createService(serviceClass: Class<S>): S {
-        return retrofitBuilder.client(clientBuilder.build()).build().create(serviceClass)
+        var usedClient: OkHttpClient? = null
+        this.okHttpClient?.let { usedClient = it } ?: run {usedClient = clientBuilder.build()}
+        return retrofitBuilder.client(usedClient).build().create(serviceClass)
     }
 
     private fun normalizeBaseUrl() {

samples/client/petstore/kotlin-retrofit2/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -18,7 +18,8 @@ import retrofit2.converter.moshi.MoshiConverterFactory
 class ApiClient(
     private var baseUrl: String = defaultBasePath,
     private val okHttpClientBuilder: OkHttpClient.Builder? = null,
-    private val serializerBuilder: Moshi.Builder = Serializer.moshiBuilder
+    private val serializerBuilder: Moshi.Builder = Serializer.moshiBuilder,
+    private val okHttpClient : OkHttpClient? = null
 ) {
     private val apiAuthorizations = mutableMapOf<String, Interceptor>()
     var logger: ((String) -> Unit)? = null
@@ -171,7 +172,9 @@ class ApiClient(
     }
 
     fun <S> createService(serviceClass: Class<S>): S {
-        return retrofitBuilder.client(clientBuilder.build()).build().create(serviceClass)
+        var usedClient: OkHttpClient? = null
+        this.okHttpClient?.let { usedClient = it } ?: run {usedClient = clientBuilder.build()}
+        return retrofitBuilder.client(usedClient).build().create(serviceClass)
     }
 
     private fun normalizeBaseUrl() {