[REQ] Kotlin generator should propose to generate open classes

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

Hello, I'm using openapi-generator to generate kotlin classes at each build of the project.
These classes are in a library, and when used in other projects, we can't inherit from the generated classes because by default they're final.

Describe the solution you'd like

Simply add an option (openClasses ?) that can be true or false.
If openClasses is true, the prefix "open" is added before the class declaration.

Describe alternatives you've considered

Work directly on generated classes, but not possible in my context.
Create classes that contains the generated class, but that makes the model really ugly.

Additional context

Nothing more to add.

No files or tests are named in the issue. Start by locating the Kotlin generator's option handling and templates for class declarations, then check how generated output is tested; done means the option can be set true or false and generated classes reflect the selected openness.