[Spring] The code-gen to have an option to generate JsonObjects instead of POJOs

Created by: javadch

In many cases, the data is stored in JSON format (e.g. MongoDB) and is delivered to the client as JSON too. In these kinds of scenarios two unnecessary JSON transformation happens; one from DB to POJO and another from the POJO to the response body. Having a JSON aware POJO can help to eliminate these two transformations. So, it is requested to have a switch in the code-gen to allow generating the model objects as JSON Aware POJOs (JOJOs). You can find more details on the issues as well as a possible solution here

openapi-generator version: 3.1.0