Add interface to add WSGI middleware

[RobbeSneyders]

As discussed in #1807.

Allowing the injection of WSGI middleware can enable easier migration from Connexion 2 to Connexion 3. The use cases are limited though, as this will only work for middleware that can work at the end of the middleware stack.

[coveralls]

coverage: 94.287% (+0.02%) from 94.271%
when pulling ed5acba on feature/wsgi_middleware
into 563fbf8 on main.