proxy setting for soap client is used for loading WSDL

For the SoapClient, a proxy can be set using $config['soapClientOptions']['proxy_host'] and $config['soapClientOptions']['proxy_port'].
This patch uses those settings to configure the proxy in the libxml stream context for fetching the WSDL.

Resolves  #176 for the 3.x branch.


Please fix [the code style issue](https://scrutinizer-ci.com/g/wsdl2phpgenerator/wsdl2phpgenerator/inspections/b45956f8-0ea6-4978-8389-a6625759ec37/issues/files/src/Wsdl2PhpGenerator/Xml/WsdlDocument.php?status=new&orderField=path&order=asc) identified by Scrutinizer.