router: change assets return type from `Route` to `AssetHandler`

So it can be easily configured when need it.

Instead of:

```
assets("/path", new AssetHandler(AssetSource.create("path")).setMaxAge(Duration.ofDays(365)));
```

Now we get:

```
  assets("/path", "path")
     .setMaxAge(Duration.ofDays(365)
```

