Add optional code challenge check for public clients

src/Entities/ClientEntityInterface.php

@@ -33,4 +33,11 @@ interface ClientEntityInterface
      * @return string|string[]
      */
     public function getRedirectUri();
+
+    /**
+     * Returns true if the client is confidential.
+     *
+     * @return bool
+     */
+    public function isConfidential();
 }