[tools/generator] Generate enum FooInterfaceVersion on server side

With this change the generator is able to detect whether an interface
follows the unstable semantics. In that case the header file on server
side looks different. An enum needs to be generated containing the
interface version. Each of the generated classes has a new method
interfaceVersion returning that enum. The ctor of the class is protected
instead of private.

So far only the header side is adjusted. The implementation currently
generates not matching code.