Add #[derive(FromRef)] (#1430)

* add `#[derive(FromRef)]`

* tests

* don't support skipping fields

probably wouldn't work at all since the whole state likely needs `Clone`

* UI tests

* changelog

* changelog link

* revert hello-world example, used for testing

* Re-export `#[derive(FromRef)]`

* Don't need to return `Result`

* use `collect` instead of quoting the iterator

* Mention it in axum's changelog

axum-macros/tests/from_ref/pass/basic.rs

@@ -0,0 +1,19 @@
+use axum_macros::FromRef;
+use axum::{Router, routing::get, extract::State};
+
+// This will implement `FromRef` for each field in the struct.
+#[derive(Clone, FromRef)]
+struct AppState {
+    auth_token: String,
+}
+
+// So those types can be extracted via `State`
+async fn handler(_: State<String>) {}
+
+fn main() {
+    let state = AppState {
+        auth_token: Default::default(),
+    };
+
+    let _: Router<AppState> = Router::with_state(state).route("/", get(handler));
+}