chore: implement LegacyPasswordAuthenticatedUserInterface::getString

This is because modern password hashing algos include the salt as part
of the hashed password.  This fixes deprecation messages such as this
one: User Deprecated: Since symfony/security-http 5.3: Returning a
string from "getSalt()" without implementing the
"Symfony\Component\Security\Core\User\LegacyPasswordAuthenticatedUserInterface"
interface is deprecated, the "UserBase\Client\Model\User" class should
implement it.

Author: boite

src/Model/User.php

@@ -4,13 +4,15 @@
 
 use LinkORB\Contracts\UserbaseRole\RoleInterface;
 use RuntimeException;
+use Symfony\Component\Security\Core\User\LegacyPasswordAuthenticatedUserInterface;
 use Symfony\Component\Security\Core\User\PasswordAuthenticatedUserInterface;
 use Symfony\Component\Security\Core\User\UserInterface as BaseUserInterface;
 
 final class User implements
     AccountContainerInterface,
     BaseUserInterface,
     LegacyAdvancedUserInterface,
+    LegacyPasswordAuthenticatedUserInterface,
     PasswordAuthenticatedUserInterface,
     PolicyContainerInterface,
     RoleInterface,
@@ -36,6 +38,7 @@ final class User implements
     private $name;
     private $password = '';
     private $roles;
+    private $salt = '';
 
     private $createdAt;
     private $lastSeenAt;
@@ -124,7 +127,7 @@ public function setPassword(string $password)
     /**
      * {@inheritdoc}
      */
-    public function getSalt()
+    public function getSalt(): ?string
     {
         return $this->salt;
     }