Add 'native' async app

[RobbeSneyders]

Next to the synchronous Flask app, we also want to provide an asynchronous app. Instead of implementing an existing async framework, I propose to implement a simple 'native' app. We can implement this app using a combination of low level starlette components and the routing lookup mechanism we already implemented in the middleware.

Instead of using the native app, the connexion middleware can also be applied to existing async frameworks to add connexion functionality.

#1395 for context.