[Kotlin] cannot configure request headers neither per request nor per ApiClient instance

Description

I am trying to use generated swagger client for REST API integration test.
During the test I need to perform some steps as admin (e.g. create a new user)
And then to act under restricted user (e.g. perform CRUD operations)
So far I can provide user credentials by modifying static field like this:

ApiClient.defaultHeaders += mapOf("Authorization" to "Bearer $token")

But this approach seems to be incorrect because I cannot run multiple tests in parallel or must only use admin credentials for all requests.

QUESTION:
Is there a way to provide credentials / configure headers per request or per ApiClient Instance?

Swagger-codegen version

2.3.1

Command line used for generation

$ java -jar swagger-codegen-cli.jar generate -i swagger.json -c kotlin-codegen-config.json -l kotlin -o ./jbt-qa-webapp-swagger-client

kotlin-codegen-config.json
{ "packageName": "io.qa", "groupId" : "io.qa", "artifactId" : "webapp-swagger-client", "artifactVersion" : "1.2.31-autogen", "enumPropertyNaming" : "UPPERCASE" }

Suggest a fix/enhancement

Java client seems to have an option to configure ApiClient instance like this

    ApiClient apiClient = new ApiClient();

    apiClient.setUsername("admin");
    apiClient.setPassword("********");
    apiClient.addDefaultHeader("Authorization", "Bearer ########")

    ActionTemplatesControllerApi apiInstance = new ActionTemplatesControllerApi(apiClient);

It would be great to have such option in Kotlin client too.

Compare the generated Kotlin ApiClient with the Java ApiClient example, focusing on how credentials and default headers are stored and passed to API instances. Implement per-instance or per-request header configuration without relying on the static defaultHeaders field, then verify that separate clients can use different credentials concurrently.