Add support for enabling `CustomSchemaApplier` while declaring route instead of a method reciever

[shambel-amare]

• the idea is if i have a User model inside my code,
  i believe implementing the CustomSchemaApplier inside my codebase makes the model cluttered.
  i think users should be able to embed CustomSchemaApplier at the point of document declaration. which separates the concern of the model and documentation.