[JAVA] Feature request: Introduce API client interface

[Shmarkus]

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 **/
}

[jonfreedman]

I think this redesign would be very beneficial - when I generate a Java client I want the model classes and some mapping between model and api endpoints. What I don't want is a load of non-applicable choices around authentication because I manage that outside the generated code.