Support PHP 8.2+

Currently if I have warnings enabled, the library emits deprecation warnings when trying to get message contents:

```
Deprecated: Creation of dynamic property Plivo\Resources\Message\Message::$uri is deprecated in /home/prosper/develop/vendor/plivo/plivo-php/src/Plivo/Resources/Message/Message.php on line 143
```

It would be great if deprecation warnings could be addressed. Thanks. 