Add support for actix_web::Either

[ManevilleF]

Using https://docs.rs/actix-web/latest/actix_web/enum.Either.html is not handled by paperclip.

Having an endpoint like:

#[api_v2_operation]
pub async my_endpoint(form: web::Either<web::Json<MyForm>, web::Form<MyForm>>) -> HttpResponse {
   let form : MyForm = match form {
        Either::Left(f) => f.0,
        Either::Right(f) => f.0,
    };
   // ...
}

Results in an error:

no variant or associated item named `update_definitions` found for enum `actix_web::Either` in the current scope