Work on refactoring statics out of the JenkinsTransportExtension and into a seperate singleton factory

and all access to factory via interface.

This completes the abstraction of the dependencies for JenkinsTransportExtension.cs and the class should now be fully testable
Nate, this class here will become the manager of instances of the JenkinsServerManager classes we will require to support projects from more than 1 jenkins server.. The next refactoring splits the "GetInstance()" method out to use a key and dictionary behind the scenes to return the correct ServerManager