Add implementation for utoipa-actix-web bindings

[juhaku]

Implements wrappers for ServiceConfig, App, Scope of actix-web. This allows users to create App with collecting paths and schemas recursively without registering them to #[openapi(...)] attribute.

Example of new supported syntax.

 use actix_web::{get, App};
 use utoipa_actix_web::{scope, AppExt};

 #[derive(utoipa::ToSchema)]
 struct User {
     id: i32,
 }

 #[utoipa::path(responses((status = OK, body = User)))]
 #[get("/user")]
 async fn get_user() -> Json<User> {
     Json(User { id: 1 })
 }

 let (_, mut api) = App::new()
     .into_utoipa_app()
     .service(scope::scope("/api/v1").service(get_user))
     .split_for_parts();

Relates #283 Relates #662
Closes #121 Closes #657