fix: revert CurrentUserChecker type

Author: attilaorosz

src/CurrentUserChecker.ts

@@ -3,4 +3,4 @@ import { Action } from './Action';
 /**
  * Special function used to get currently authorized user.
  */
-export type CurrentUserChecker = <User = any>(action: Action) => Promise<User | null> | User | null;
+export type CurrentUserChecker = (action: Action) => Promise<any> | any;