[feature] allow middleware to transform a response

[ghost]

description

i think kemal should allow middleware to transform responses from routes in addition to transforming requests. it would add a fair bit of power to be able to throw a specific exception in a route and have a middleware catch that exception and return a certain error page

usecases:

• validate in the route, throw a ValidationError, which the middleware catches and responds with an HTTP 400
• transform the response body with additional content or processing
• move certain common processing from routes to middleware