Allow specifying access modifiers for generated classes

[kirides]

First off, sorry for feature bombing

It would be very handy to be able to specify the access modifier for the generated DTOs and Clients.

backstory:

In our project, we have modularized everything and try to keep our public Api surface as small as possible.
We define most of our classes as internal unless they explicitly are needed in other places.

All HTTP calls a module needs and all their DTOs reside in the module and never leave it's scope.

[christianhelle]

@kirides thank you for your input

Yes, it is possible to generate the interface API and contract types as internal. I can expose this in some CLI arguments, but the default will be public

[christianhelle]

@kirides I have implemented this and will release it tonight (CEST)