Implement middleware for &T: Middleware<_>

That allows to pass middlewares with their configuration around at
application setup time without having to clone them.

poem/src/middleware/mod.rs

@@ -301,6 +301,14 @@ impl<E: Endpoint> Middleware<E> for () {
     }
 }
 
+impl<E: Endpoint, T: Middleware<E>> Middleware<E> for &T {
+    type Output = T::Output;
+
+    fn transform(&self, ep: E) -> Self::Output {
+        T::transform(self, ep)
+    }
+}
+
 /// A middleware that combines two middlewares.
 pub struct CombineMiddleware<A, B, E> {
     a: A,