[4.x] Make impersonation tokens require stateful guards (#935)

* Throw an exception on attempt to create impersonation token with a non-stateful guard

* Test that impersonation tokens can only be created with a stateful guard

* Fix code style (php-cs-fixer)

* Escape backslashes in the exception's message

Co-authored-by: Samuel Štancl <samuel.stancl@gmail.com>

* Make the exception only about requiring a stateful guard

Co-authored-by: PHP CS Fixer <phpcsfixer@example.com>
Co-authored-by: Samuel Štancl <samuel.stancl@gmail.com>

src/Exceptions/StatefulGuardRequiredException.php

@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Stancl\Tenancy\Exceptions;
+
+use Exception;
+
+class StatefulGuardRequiredException extends Exception
+{
+    public function __construct(string $guardName)
+    {
+        parent::__construct("Cannot use a non-stateful guard ('$guardName'). A guard implementing the Illuminate\\Contracts\\Auth\\StatefulGuard interface is required.");
+    }
+}