Introduce RequestUtil to encapsulate getting/setting actor on requests(#2449)

src/Extend/ThrottleApi.php

@@ -26,7 +26,7 @@ class ThrottleApi implements ExtenderInterface
      *
      * The callable can be a closure or invokable class, and should accept:
      *   - $request: The current `\Psr\Http\Message\ServerRequestInterface` request object.
-     *               `$request->getAttribute('actor')` can be used to get the current user.
+     *               `\Flarum\Http\RequestUtil::getActor($request)` can be used to get the current user.
      *               `$request->getAttribute('routeName')` can be used to get the current route.
      * Please note that every throttler runs by default on every route.
      * If you only want to throttle certain routes, you'll need to check for that inside your logic.