Update AllowedFilter.php to include getFilterClass function (#909)

* Update AllowedFilter.php to include getFilterClass

requires if you need to rebuild the filter on the fly

* Update AllowedFilter.php -> getFilterClass return type

src/AllowedFilter.php

@@ -118,6 +118,11 @@ public static function custom(string $name, Filter $filterClass, $internalName =
         return new static($name, $filterClass, $internalName);
     }
 
+    public function getFilterClass(): \Spatie\QueryBuilder\Filters\Filter
+    {
+        return $this->filterClass;
+    }
+
     public function getName(): string
     {
         return $this->name;