ENH Provide onBeforeRenderHolder extension hook.

GridField has onBeforeRender in its Field method, but that hardly ever gets called.
This commit adds the onBeforeRenderHolder extension hook that is normally available via FormField::FieldHolder().

src/Forms/GridField/GridField.php

@@ -438,6 +438,8 @@ class GridField extends FormField
      */
     public function FieldHolder($properties = [])
     {
+        $this->extend('onBeforeRenderHolder', $this, $properties);
+        
         $columns = $this->getColumns();
 
         $list = $this->getManipulatedList();