Allow alt_response on MethodView

Nobody has claimed this yet.

It would be a nice enhancement to allow the usage of the `alt_response`-Decorator on `MethodView`-classes.
It could act as a shortcut to decorating every endpoint of the view with `alt_response`.

An example use case would be a custom converter that rejects any pet_id not present in the pet database.
Since its declared in the `route`-decorator, every endpoint will raise 404 if the pet was not found:
```python3

@blp.route('/')
@blp.alt_response(404, ErrorSchema)
class PetsById(MethodView):

@blp.response(200, PetSchema)
def get(self, pet_id):
"""Get pet by ID"""
return Pet.get_by_id(pet_id)

@blp.response(204)
def delete(self, pet_id):
"""Delete pet"""
Pet.delete(pet_id)
```

Start by tracing how the alt_response and route decorators are applied to a Flask MethodView and its endpoint methods. Check the provided PetsById example, including the custom converter that raises 404, and determine how one class-level alt_response should affect every endpoint. Done means the class-level usage produces the intended documented 404 response for each endpoint.

Written by the indexing model from the issue text.