Derive for OpenApiResponderInner

[vklachkov]

Description

This PR adds derive macro for the OpenApiResponderInner trait. It's fully compatible with Rocket's Response derive macro.

Also, to reduce code generation, add set_description function to utils.

This pull request does not add any usage examples.

Codegen Examples

1. Simple enum:

   #[derive(Responder, OpenApiResponder)]
   enum ServerResult {
       A(String),
       B(Json<u32>),
   }
   

   Macro expansion

   impl ::rocket_okapi::response::OpenApiResponderInner for ServerResult {
       fn responses(
           gen: &mut ::rocket_okapi::gen::OpenApiGenerator,
       ) -> ::rocket_okapi::Result<::rocket_okapi::okapi::openapi3::Responses> {
           let mut responses = ::rocket_okapi::okapi::openapi3::Responses::default();
           responses.responses.extend(
               {
                   let mut r =
                       <String as ::rocket_okapi::response::OpenApiResponderInner>::responses(gen)?;
                   r
               }
               .responses,
           );
           responses.responses.extend(
               {
                   let mut r =
                       <Json<u32> as ::rocket_okapi::response::OpenApiResponderInner>::responses(gen)?;
                   r
               }
               .responses,
           );
           Ok(responses)
       }
   }
   

2. Generic struct:

   #[derive(Responder, OpenApiResponder)]
   #[response(status = 404, content_type = "html")]
   struct NotFoundHtml<T>(T);
   

   Macro expansion

   impl<T: ::rocket_okapi::response::OpenApiResponderInner> ::rocket_okapi::response::OpenApiResponderInner for NotFoundHtml<T> {
       fn responses(
           gen: &mut ::rocket_okapi::gen::OpenApiGenerator,
       ) -> ::rocket_okapi::Result<::rocket_okapi::okapi::openapi3::Responses> {
           let mut responses = ::rocket_okapi::okapi::openapi3::Responses::default();
           responses.responses.extend(
               {
                   let mut r = <T as ::rocket_okapi::response::OpenApiResponderInner>::responses(gen)?;
                   ::rocket_okapi::util::set_status_code(&mut r, 404u16)?;
                   ::rocket_okapi::util::set_content_type(&mut r, "text/html; charset=utf-8")?;
                   r
               }
               .responses,
           );
           Ok(responses)
       }
   }
   

Issues

Fixes https://github.com/GREsau/okapi/issues/146.