[TOOLS] Add explicit return types to fix deprecation warnings raised by PHPUnit

plugins/OAuth2/Entity/Client.php

@@ -187,7 +187,7 @@ class Client extends Entity implements ClientEntityInterface
     // @codeCoverageIgnoreEnd
     // }}} Autocode
 
-    public function getIdentifier()
+    public function getIdentifier(): string
     {
         return $this->getId();
     }