Situation: there is a local network, and I want to setup an nginx as caching proxy that will serve content taken from servers in the Internet to users in local network.
Problem appears when access to the Internet from that local network is protected by some proxy server with authorization.

Question: is there a way to configure nginx to pass all his requests through that proxy supplying login and password? Or do I need to setup some another application between nginx and proxy?

And what if nginx is placed behind regular proxy server, without authorization?
Is there a solution for such case?