Allow to specify the generated part (model, api...)

Created by: Zomzog

This fix #13830 (closed)

The idea behind the PR is to allow more control to the user for selecting the generation. Currently, we can generate only some models by names. But here we can generate all the models without generating the API.

The code is quite weird because I've kept the initial behaviors. So the behavior changes only if the new properties are used.

If the change is OK I can try to update Gradle and maven plugins to use them.

Ping of the core team because it's a global behavior @wing328 @jimschubert @cbornet @jmini @etherealjoy@spacether

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