Enhance configurability of the OkHttpClient used by Java+retrofit2

Description

The class ApiClient generated for Java+retrofit2 is not easily configurable.
The default constructor of the class calls the method createDefaultAdapter().
This method configures the "adapter builder" that is later used go create the services as shown below:

  public void createDefaultAdapter() {
    json = new JSON();
    okBuilder = new OkHttpClient.Builder();

    String baseUrl = "https://localhost/someco/contracts-api";
    if (!baseUrl.endsWith("/"))
      baseUrl = baseUrl + "/";

    adapterBuilder = new Retrofit
      .Builder()
      .baseUrl(baseUrl)
      .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
            .addConverterFactory(ScalarsConverterFactory.create())
      .addConverterFactory(GsonCustomConverterFactory.create(json.getGson()));
  }

  public <S> S createService(Class<S> serviceClass) {
    return adapterBuilder
      .client(okBuilder.build())
      .build()
      .create(serviceClass);
  }

Swagger-codegen version

swagger-codegen-maven-plugin 2.3.1

Command line used for generation

<plugin>
    <groupId>io.swagger</groupId>
    <artifactId>swagger-codegen-maven-plugin</artifactId>
    <version>2.3.1</version>
    <executions>
        <execution>
            <goals>
                <goal>generate</goal>
            </goals>
            <configuration>
                <output>${project.basedir}</output>
                <!-- specify the swagger yaml -->
                <inputSpec>${project.basedir}/api-editor/api/swagger/swagger.yaml</inputSpec>
                <!-- target to generate java client code -->
                <language>java</language>
                <generateApiTests>true</generateApiTests>
                <generateModelTests>true</generateModelTests>
                <configOptions>
                    <dateLibrary>joda</dateLibrary>
                    <invokerPackage>com.someco.client.invoker</invokerPackage>
                    <apiPackage>com.someco.client.api</apiPackage>
                    <modelPackage>com.someco.client.model</modelPackage>
                    <sourceFolder>src/gen/java</sourceFolder>
                    <serializableModel>true</serializableModel>
                    <hideGenerationTimestamp>true</hideGenerationTimestamp>
                </configOptions>
                <!-- override the default library to retrofit2 -->
                <library>retrofit2</library>
            </configuration>
        </execution>
    </executions>
</plugin>

Steps to reproduce

Generate the client code using maven and the configuration above

Suggest a fix/enhancement

With the current implementation it seems impossible, for example, to configure a reusable OkHttpClient that is configured with a proper connection pool.
Because the method createDefaultAdapter() needs to configure private members, it cannot be overridden by a super class and so its behaviour cannot be modified using inheritance.

Couldn't be better to make it possible to execute a code like:

ConnectionPool pool = new ConnectionPool(poolSize, keepAlive, TimeUnit.SECONDS);
OkHttpClient client = apiClient.getOkBuilder().connectionPool(pool).build();
apiClient.getAdapterBuilder().baseUrl(apiPath).client(client);
apiClient.build(); // this builds the adapter (a "Retrofit" object) using the adapterBuilder.

Start by locating the Java retrofit2 generation entry points that produce ApiClient, then review createDefaultAdapter() and createService() to understand the current builder lifecycle. The work is complete when generated clients allow callers to configure a reusable OkHttpClient and Retrofit adapter without modifying generated code; the issue does not name specific template files or tests.