Allow for relation key to be an enum (#50311)

* Allow for relation key to be an enum

If a foring key is being cast to an enum calling ->sync() will cause a TypeError: Illegal offset type.

This resolves the issue by getting the value before using it as an array key.

* Update InteractsWithPivotTable.php

---------

Co-authored-by: Taylor Otwell <taylor@laravel.com>

src/Illuminate/Database/Eloquent/Relations/Concerns/InteractsWithPivotTable.php

@@ -2,6 +2,7 @@
 
 namespace Illuminate\Database\Eloquent\Relations\Concerns;
 
+use BackedEnum;
 use Illuminate\Database\Eloquent\Collection;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\Pivot;
@@ -154,6 +155,10 @@ protected function formatRecordsList(array $records)
                 [$id, $attributes] = [$attributes, []];
             }
 
+            if ($id instanceof BackedEnum) {
+                $id = $id->value;
+            }
+
             return [$id => $attributes];
         })->all();
     }