fixes #8484 - make SmartProxyAuth concern more useful to plugins

A lot of the `alias_method_chain` trickery in foreman_salt is hacking away at this concern, and I'd love to get rid of it. I hope this isn't too disruptive a change for 1.8.

I know at least foreman_chef will need a change to implement `authorized_smart_proxy_features` in the FactImporter (eventually, I'd like to do the same to ReportImporter).