[JAVA] Feature request: Introduce API client interface

Description

At the moment Swagger generates concrete implementation of ApiClient which is then passed to my actual API client. It would be beneficial to introduce ApiClientInterface that the custom client then accepts, which would allow the developer to write integration tests against Mock APIs.

At the moment, the only option is to generate stub server with dummy data and change the endpoint when doing integration tests.

Suggest a fix/enhancement

public class ApiClient implements ApiInterface {
    /** same as now **/
}

public class DefaultApi {
    private ApiInterface apiClient;

    public DefaultApi(ApiInterface apiClient) {
        this.apiClient = apiClient;
    }
    /** same as now **/
}

No files or tests are named. Start by locating the Java generator entry points and the templates that produce ApiClient and DefaultApi, then check how generated clients are wired together. Done means generated Java clients can accept an API client interface so integration tests can substitute mock APIs.