Drop Guzzle requirement and replace with any PSR-18 HttpClient

The current version of this SDK requires Guzzle. My application uses Symfony's HttpClient (which allows for automatic retries and integration with the debug toolbar, compelling reasons for me to use it).

Rather than recommending this library switch, better would be use only require that any PSR-18 http client be installed, and use it.

Here's an article about how and why: https://medium.com/php-fig/psr-18-the-php-standard-for-http-clients-3254c58752bb

At a minimum, even if Guzzle is installed, the developer should be able to choose what client they want to use for the requests.

Thanks!