Start of SSH2 Agent Forwarding implementation

As I am rather keen on having this work I've written code to support a SSH2 Agent Forwarding implementation. This PR does enable this functionality but as I am not super familiar with PHP or the design of this library I'm expecting some feedback or comments. 

This PR will address this current OPEN issue / feature request. 

https://github.com/phpseclib/phpseclib/issues/505
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