[REQ] The java play framework generator should support builder pattern models

Created by: gitterrost4

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

The java-play-framework generator will generate models as mutable as it gets. This makes it clunky to work with them (especially in the functional environment of java8+) and leads to potential mistakes.

Describe the solution you'd like

I'd suggest an option to create the models immutable with a builder pattern for creation.

Additional context

I do have a branch with the necessary code to implement this, but I am unsure if I can submit it as a pull request or if I need to wait for feedback in this issue.