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!

檢查相依性設定和 SDK 的請求路徑，以找出在哪些地方需要 Guzzle，以及如何選取 HTTP 用戶端。完成的標準是 SDK 可以使用任何 PSR-18 用戶端，包括 Symfony's HttpClient，同時在已安裝 Guzzle 時保留選取用戶端的方式。

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