[REQ] Kotlin Generator: allows generated APIs/Client Classes to be "kotlin open" (non final)

[f-aubert]

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

Using Kotlin Open API Generator together with EnableAspectJAutoProxy annotation doesn't actually work, since some of the generated classes are not open (=final) and thus not proxyable.

Describe the solution you'd like

Making the APIs/Clients classes by default open or allowing a configParameter to enable or disable the feature would fix this technology combination dead-end

Describe alternatives you've considered

Using kotlin all-open-compiler-plugin, but it requires the classes to have an annotation.

Please forgive me if I'm missing something obvious.