refactor(core): centralize reserved system user identity

Signed-off-by: Sebastian Krupinski <krupinski01@gmail.com>

lib/Provider.php

@@ -7,8 +7,8 @@ namespace KTXM\AuthenticationProviderMail;
 use KTXC\Service\UserAccountsService;
 use KTXF\Mail\Object\Address;
 use KTXF\Mail\Object\MessagePropertiesBaseInterface;
-use KTXF\Mail\Provider\ProviderBaseInterface;
 use KTXF\Mail\Submission\EntitySubmitResult;
+use KTXF\Resource\SystemIdentity;
 use KTXF\Security\Authentication\AuthenticationProviderAbstract;
 use KTXF\Security\Authentication\ProviderContext;
 use KTXF\Security\Authentication\ProviderResult;
@@ -264,7 +264,7 @@ class Provider extends AuthenticationProviderAbstract
             // Submit through the system mail route for the authentication channel
             $result = $this->mailManager->entitySubmit(
                 $tenantId,
-                ProviderBaseInterface::USER_SYSTEM,
+                SystemIdentity::USER,
                 'authentication@system',
                 null,
                 [