Is marshall_with unnecessary boilerplate?

In principal, it seems like `marshall_with` is unnecessary boilerplate.

If we could attach a `marshmallow.Schema` to classes (e.g. database models) returned by Flask routes, `flask-apispec` should be able to hook into Flask's post-request machinery to infer the relevant schema and serialize them accordingly.

I'd be willing to try to implement this it's a good idea and if someone could give me some pointers on how to do it.

Start by tracing the existing `marshall_with` behavior and Flask's post-request machinery, then examine how `marshmallow.Schema` could be attached to classes returned by Flask routes. Done would mean a reviewed design and implementation that infers the relevant schema and serializes those responses without requiring `marshall_with`.

Written by the indexing model from the issue text.