[kotlin] OkHttpClient is not customizable.

Description

There is no chance to access response headers because

1. It is impossible to add interceptors to OkHttpClient
2. headers on ApiInfrastructureResponse is ignored .

open class ApiClient(val baseUrl: String) {
    companion object {
        @JvmStatic
        val client : OkHttpClient = OkHttpClient() // No chance to customize OkHttpClient
    }

abstract class ApiInfrastructureResponse<T>(val responseType: ResponseType) {
     abstract val statusCode: Int
     abstract val headers: Map<String,List<String>> // Set, but not be used anywhere.
}

// api.mustache
return when (response.responseType) {
    // Only use data property, headers is ignored.
    ResponseType.Success -> {{#returnType}}(response as Success<*>).data as {{{returnType}}}{{/returnType}}{{^returnType}}Unit{{/returnType}}
    ResponseType.Informational -> TODO()
    ResponseType.Redirection -> TODO()
    ResponseType.ClientError -> throw ClientException((response as ClientError<*>).body as? String ?: "Client error")
    ResponseType.ServerError -> throw ServerException((response as ServerError<*>).message ?: "Server error")
    else -> throw kotlin.IllegalStateException("Undefined ResponseType.")
}

Swagger-codegen version

2.3.1

Swagger declaration file content or url

Command line used for generation

Steps to reproduce

Related issues/PRs

https://github.com/swagger-api/swagger-codegen/issues/7602

Suggest a fix/enhancement

I think it would be better to set user customized OkHttpClient.Builder to ApiClient.Companion instead of OkHttpClient. I will try to fix these.

Start with the Kotlin ApiClient and ApiInfrastructureResponse examples, then inspect api.mustache where the response is converted to the return value. Trace how OkHttpClient is created and how response headers are represented. Done means a generated client can use a customized OkHttpClient.Builder and response headers are no longer ignored.