add dyn_reply example

examples/dyn_reply.rs

@@ -0,0 +1,18 @@
+#![deny(warnings)]
+use warp::{http::StatusCode, Filter};
+
+async fn dyn_reply(word: String) -> Result<Box<dyn warp::Reply>, warp::Rejection> {
+    if &word == "hello" {
+        //a cast is needed for now, see https://github.com/rust-lang/rust/issues/60424
+        Ok(Box::new("world") as Box<dyn warp::Reply>)
+    } else {
+       Ok(Box::new(StatusCode::BAD_REQUEST) as Box<dyn warp::Reply>)
+    }
+}
+
+#[tokio::main]
+async fn main() {
+    let routes = warp::path::param().and_then(dyn_reply);
+
+    warp::serve(routes).run(([127, 0, 0, 1], 3030)).await;
+}