PHP 8.2+ Deprecation: Declare $httpClient property #16

Description

The ThothApi\Rest\Client class dynamically creates the $httpClient property without declaring it, which triggers a deprecation warning in PHP 8.2+: https://github.com/SteelWagstaff/thoth-client-php/blob/8b7b9aa077c7cdc6fb7de132454abd2df9cedee5/src/Rest/Client.php#L20.

Error

Deprecated: Creation of dynamic property ThothApi\Rest\Client::$httpClient is deprecated in /path/to/Client.php on line 18

Addressed by #15