Apply a [`tower::Layer`] to the router that will only run if the request matches
a route.
Note that the middleware is only applied to existing routes. So you have to
first add your routes (and / or fallback) and then call `layer` afterwards. Additional
routes added after `layer` is called will not have the middleware added.
This works similarly to [`MethodRouter::layer`] except the middleware will only run if
the request matches a route. This is useful for middleware that return early
(such as authorization) which might otherwise convert a `405 Method Not Allowed` into a
`401 Unauthorized`.
# Example
```rust
use axum::{
routing::get,
Router,
};
use tower_http::auth::RequireAuthorizationLayer;
let app = Router::new().route(
"/foo",
get(|| async {})
.route_layer(RequireAuthorizationLayer::bearer("password"))
);
// `GET /foo` with a valid token will receive `200 OK`
// `GET /foo` with a invalid token will receive `401 Unauthorized`
// `POST /FOO` with a invalid token will receive `405 Method Not Allowed`
# async {
# axum::Server::bind(&"".parse().unwrap()).serve(app.into_make_service()).await.unwrap();
# };
```