Cast client id and client secret to string for compatibility with PHP…

…8.2 (#20)

* Cast client id and client secret to string
* Linter changes

---------

Co-authored-by: Piotr Matras <piotr.matras@creativestyle.pl>
Co-authored-by: Šaltanovas Aivaras <zumbass@gmail.com>
Co-authored-by: Šaltanovas Aivaras <aivaras.saltanovas@kevin.eu>

Gateway/Config/Config.php

@@ -82,14 +82,20 @@ public function getActive()
         return $this->getValue('active');
     }
 
+    /**
+     * @return string
+     */
     public function getClientId()
     {
-        return $this->getValue('client_id');
+        return (string) $this->getValue('client_id');
     }
 
+    /**
+     * @return string
+     */
     public function getClientSecret()
     {
-        return $this->getValue('client_secret');
+        return (string) $this->getValue('client_secret');
     }
 
     public function getSignature()