[REQ] Allow JavaJaxRsSpec Generator to generate asynchronous APIs

Created by: prodanov

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

I'm using the model generation and API generation of the JaxRsSpec generator and I like it a lot as it gives me a lot of freedom in implementing the server. Quite Specific I'm using it with Quarkus and in order to have asynchronous routes in quarkus you API has to return CompletionStage. CompletionStage is a class from the JDK. A simplified example could look like:

`import java.util.concurrent.CompletionStage;

interface FruitApi { CompletionStage getFruit(); }`

It would be ok to have an "async" flag in the config that will generate those types of APIs. Maybe with some other options like in the spring generator.