This also makes the use of a Proxy instance explicit in the Getdown code,
rather than trying to thread things through the proxy-related system
properties.
It introduces a ProxyAuth SPI which allows an app installation to save proxy
credentials in whatever way it deems fit. Otherwise the credentials are unsaved
and the user effectively has to enter them every time Getdown attempts to
access the internet.
This is obviously a terrible design, but there's no good (cross-platform) way
to securely store a username and password. So anyone using this feature is
likely going to need to provide an implementation of ProxyAuth that does
whatever auth storage they deem appropriate.
Adapted from PR from @ThomasG-AI, thanks!
Michael Bayne
ae129fad6f