(#16635) add util/http_proxy with http_proxy_{host,port} methods


In 3.x, the pkgdmg provider no longer honors http_proxy_host or http_proxy_port
settings or environment variables. This takes the methods to do this from
puppet/forge/repository and puts them in puppet/util/http_proxy, and updates
repository.rb and pkgdmg.rb to use it.

Also, add some tests.