Add proxy support

[KevinBaland]

Add proxy support by adding a "proxy" property to the $settings array.

Example :

$settings = [
      'serverUrl' => 'https://xxxxxx.crmX.dynamics.com',
      'username' => 'xxxxxxxxxxxxxxxxxxxxxxxxxxxx',
      'password' => 'xxxxxxxxxxxxxxxxxxxxxxxxxxxx',
      'authMode' => 'OnlineFederation',
      'proxy' => 'your_proxy_host:your_proxy_port',
    ];

$serviceSettings = new Settings($settings);