Move axum-handle-error-extract into axum (#534)

* Move `axum-handle-error-extract` into axum

With 0.4 underway we can now nuke `axum-handle-error-extract` and move
its code directly into axum.

So this replaces the old `HandleErrorLayer` with one that supports async
functions and extractors.

* changelog

* fix CI

examples/key-value-store/src/main.rs

@@ -124,7 +124,7 @@ fn admin_routes() -> Router {
         .layer(RequireAuthorizationLayer::bearer("secret-token"))
 }
 
-fn handle_error(error: BoxError) -> impl IntoResponse {
+async fn handle_error(error: BoxError) -> impl IntoResponse {
     if error.is::<tower::timeout::error::Elapsed>() {
         return (StatusCode::REQUEST_TIMEOUT, Cow::from("request timed out"));
     }