[REQ] [Java] [native] Make generated ApiClient friendlier for extending

Is your feature request related to a problem? Please describe.

I am using Java generation with library = "native" and there are some things (ObjectMapper, HttpClient.Builder, ...) I would like to customize.

Describe the solution you'd like

I would like to add a new generated constructor that takes some of the values as parameters.
The default constructor will call the new constructor.
Backward compatibility will be kept.
I will have compilation validation as an advantage in case future changes are made to the auto generated ApiClient.

Describe alternatives you've considered

a. I can use the setters but that issues with this approach are:

1. The code running in the constructor is thrown away (ObjectMapper creation is considered expensive).
2. If we init variables in one place and override in another, it makes the code less readable.

b. I can use a custom template but than I will need to follow the template changes every time I upgrade openapi-generator so I won't miss any important updates.

Additional context

I am planning something like this:

  public ApiClient() { // Existing ctor refactored
    this(HttpClient.newBuilder(), createDefaultObjectMapper(), getDefaultBaseUri()); // Using new ctor
    interceptor = null;
    readTimeout = null;
    responseInterceptor = null;
  }

  public ApiClient(Builder builder, ObjectMapper mapper, String baseUri) { // New ctor
    this.builder = builder;
    this.mapper = mapper;
    updateBaseUri(baseUri);
  }

  public static ObjectMapper createDefaultObjectMapper() { // New method with old logic
    ObjectMapper mapper = new ObjectMapper();
    mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
    mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    mapper.configure(DeserializationFeature.FAIL_ON_INVALID_SUBTYPE, false);
    mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
    mapper.enable(SerializationFeature.WRITE_ENUMS_USING_TO_STRING);
    mapper.enable(DeserializationFeature.READ_ENUMS_USING_TO_STRING);
    mapper.registerModule(new JavaTimeModule());
    JsonNullableModule jnm = new JsonNullableModule();
    mapper.registerModule(jnm);
    return mapper;
  }

  public static String getDefaultBaseUri() { // New method
    return "https://demo-nokia.omnyfy.com/rest/all";
  }

  public void updateBaseUri(String baseUri) { // New method, old logic
    URI uri = URI.create(baseUri);
    scheme = uri.getScheme();
    host = uri.getHost();
    port = uri.getPort();
    basePath = uri.getRawPath();
  }

Start by locating the Java generator template for the native-library ApiClient and the compilation-validation path; the issue names no files or tests. Done means the default constructor remains backward compatible while a new constructor and reusable default-value methods allow customization of the builder, mapper, and base URI.