feat: change user to user_id as part of payload (#118)

Author: totalimmersion

lib/GetStream/StreamChat/Channel.php

@@ -64,7 +64,7 @@ public function getCID(): string
      */
     private static function addUser(array $payload, string $userId)
     {
-        $payload["user"] = ["id" => $userId];
+        $payload["user_id"] = $userId;
         return $payload;
     }