public class ProxySettings
Responsible for all things related to proxies. This includes storing the manually-set proxy
host and port (if any), deciding what proxy, if any, to use for a given request
(determined by manual or system settings), and serving up username/password credentials if required
for an authenticating proxy.
The one thing this class does not do is handle prompting the user for username and password
if necessary. That is handled in Mathematica code triggered by a 407 HTTP status.
The HttpHandler instance holds one instance of this class and uses it when necessary or passes
it to other classes that need to use it.