[BUG][cpp-restsdk client] Multiple response schemas not supported

Description

I believe OpenAPI allows us to create APIs with multiple response Schemas, which was debated to death in this ticket: https://github.com/OAI/OpenAPI-Specification/issues/270. I'm asking for support in cpp-restsdk, since my API is born of some compexity that we can't easily handle using the current implementation effectively.

openapi-generator version

openapi-generator 5.0.1.0

OpenAPI declaration file content or url

  /complexResponse:
    get:
      summary: Checks the status of a feature
      responses:
        200:
          description: The feature has completed
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/didStuff'
        202:
          description: feature is at work
        412:
          description: you are not ready for this feature
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/errorObj'
components:
  schemas:
    didStuff:
      type: string
    errorObj:
      type: object
      properties:
        error:
          type: string

Steps to reproduce

So here's some googleTest I'd have to write to test the 3 responses:

TEST_F(tryMe, complexApiTest) 
{
	std::shared_ptr<ApiConfiguration> apiconfiguration = std::make_shared<ApiConfiguration>();
	apiconfiguration->setBaseUrl(utility::conversions::to_string_t("http://127.0.0.1/api/default"));
	std::shared_ptr<ApiClient> apiclient = std::make_shared<ApiClient>(apiconfiguration);
	std::shared_ptr<myApi> api = std::make_shared<myApi>(apiclient);
	try
	{
		api->complexResponse().then([&](std::shared_ptr<didStuff> result){
                      std::cout << "well the main stuff worked" << std::endl;
                }).wait();
	}
	catch (const ApiException& e)
	{
		std::cout << e.what() << std::endl;
		std::string s(std::istreambuf_iterator<char>(*e.getContent()), {});
		utility::string_t localVarResponse = utility::conversions::to_string_t(s);
		web::json::value localVarJson = web::json::value::parse(localVarResponse);
		std::shared_ptr<PreconditionsResponse> test = std::make_shared<errorObj>();
		test->fromJson(localVarJson);
                std::cout << "our 412 was presumably hit" << std::endl;
	}
	catch (const web::json::json_exception& e)
	{
		std::cout << "we can only assume that we returned a 202 which has an empty response" << std::endl;
	}
}

Related issues/PRs

https://github.com/OAI/OpenAPI-Specification/issues/270

Suggest a fix

Somehow creating multiple pplx tasks in the api-source.mustache might work for us. Overriding {{#operation}} for each schema would be an interesting find. If we would have to put in n amount task callbacks as our swagger specifies, that would be ideal.

BUT AT THE VERY LEAST, moving the response code that throws an ApiException needs to be raised from 400 to 500 in api-source.mustache line 279.

Start with modules/openapi-generator/src/main/resources/cpp-rest-sdk-client/api-source.mustache, especially the response handling around line 279, and compare it with the supplied OpenAPI declaration. Use the generated client and the proposed GoogleTest scenario to examine 200, 202, and 412 responses. Done means the generated cpp-restsdk client supports the declared response schemas and status codes with regression coverage.