Support for proxy/no-proxy

[igittigitt]

Currently the underlying Guzzle HTTP-Client use the OS settings to use a proxy or not. This can lead to unexpected results (e.g. 503 errors). I suggest to add a switch to also supply a Guzzle-like array of proxy-settings as mentioned in the Guzzle docs: https://docs.guzzlephp.org/en/latest/request-options.html#proxy

[igittigitt]

As a suggestion add this class variable to DcxApiClient:
/** @var array Proxy to use (default is to void using a proxy) */
protected $customHttpProxy = [ 'no' => ['*'] ];

and the "proxy" option to the function getRequestOptions:
'proxy' => $this->customHttpProxy,