Code generator: Generate typed Feature objects

To improve static type checking capabilities, especially metadata-handling in server-side code, it would be great to have type-annotated feature objects. This could be implemented via stub files like greetingprovider_feature.pyi.

Examples:

class GreetingProviderFeature(Feature):
    SayHello: UnobservableCommand[SayHello_Parameters, SayHello_Responses]
    StartYear: UnobservableProperty[int]

class DelayProviderFeature(Feature):
    RandomNumber: UnobservableProperty[int]
    Delay: Metadata[int]