PHP 8.2 Compatibility of Dispatcher Class

If you use the Dispatcher in a PHP8.2 Runtime you get a Deprecation Notice:

```
NOTICE: PHP message: PHP Deprecated:  Creation of dynamic property AdvancedJsonRpc\Dispatcher::$mapper is deprecated 
```

To avoid this I will create a merge-Request that adds the missing property and imports `\JsonMapper`-Class. Maybe correlates to #57