text: Title: Allow alt_response on MethodView
  Body: 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('/<object_id(must_exist=True):pet_id>')
  @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)
  ```