Add async-graphql example (#93)

Fixes https://github.com/tokio-rs/axum/issues/68

examples/async-graphql/main.rs

@@ -0,0 +1,35 @@
+mod starwars;
+
+use async_graphql::http::{playground_source, GraphQLPlaygroundConfig};
+use async_graphql::{EmptyMutation, EmptySubscription, Request, Response, Schema};
+use axum::response::IntoResponse;
+use axum::{prelude::*, AddExtensionLayer};
+use starwars::{QueryRoot, StarWars, StarWarsSchema};
+
+async fn graphql_handler(
+    schema: extract::Extension<StarWarsSchema>,
+    req: extract::Json<Request>,
+) -> response::Json<Response> {
+    schema.execute(req.0).await.into()
+}
+
+async fn graphql_playground() -> impl IntoResponse {
+    response::Html(playground_source(GraphQLPlaygroundConfig::new("/")))
+}
+
+#[tokio::main]
+async fn main() {
+    let schema = Schema::build(QueryRoot, EmptyMutation, EmptySubscription)
+        .data(StarWars::new())
+        .finish();
+
+    let app = route("/", get(graphql_playground).post(graphql_handler))
+        .layer(AddExtensionLayer::new(schema));
+
+    println!("Playground: http://localhost:3000");
+
+    hyper::Server::bind(&"0.0.0.0:3000".parse().unwrap())
+        .serve(app.into_make_service())
+        .await
+        .unwrap();
+}