add flags to permissions groups

Flags can be added to permissions group to store some metadata.

An interface `PermissionsGroupFlagProvider` allow to add some flag to the form which create/
edit permissions groups.

Form/Utils/PermissionsGroupFlagProvider.php

@@ -0,0 +1,18 @@
+<?php
+
+
+namespace Chill\MainBundle\Form\Utils;
+
+/**
+ *
+ * 
+ */
+interface PermissionsGroupFlagProvider
+{
+    /**
+     * Return an array of flags
+     * 
+     * @return string[] an array. Keys are ignored.
+     */
+    public function getPermissionsGroupFlags(): array;
+}