fix: Add optional handlers for Accounting, Domain, and RootServer

Introduced nullable properties for Accounting, Domain, and RootServer handlers in the Client class. These additions prepare the Client for expanded functionality and modular handling of related operations. No impact on existing behavior without explicit handler usage.

Author: efebagri

src/Client.php

@@ -16,6 +16,10 @@ class Client
     private Credentials $credentials;
     private string $token;
 
+    private ?Accounting $accountingHandler = null;
+    private ?Domain $domainHandler = null;
+    private ?RootServer $rootServerHandler = null;
+
     /**
      * Client constructor.
      *