Use this macro to generate serde Serialize/Deserialize impls in addition
to an ApiBody impl that can hand back information about the shape of the
type.
Use this macro to generate an Into<ApiError> implementation for your custom error
type. Your custom error type needs to implement Debug and Display in order to
derive ApiError. Display in particular determines what the error message will be.
You can then use attribtues to set the status code, and decide on whether the
error message will be internal-only or external.