Add support for MacOS

[banaru4]

This adds support for MacOS #1 by reading from the preferences.plist file found in /Library/Preferences/SystemConfiguration/
Proxy exceptions are distinct for different network adapters. The ProxyConfigInternal structure contains the network adapter name for which the proxy is configured.

[awakecoding]

@mattico any update on this? I work with @banaru4 and we are currently using proxy-config with patches from this PR in a project we'd like to publish to crates.io (https://github.com/devolutions/siquery-rs/). Since we can't publish with references to a git repository, would it be possible to let us know if this PR has a chance of being merged soon? Since this project was inactive for two years, I assume it could be no longer actively maintained, so we offer our help in maintaining it officially.

[mattico]

Hi all, sorry for the late response. As you may have guessed I no longer need this library for my own use, so I haven't had any motivation to work on it.

I'd be happy to add you as an owner to crates.io/proxy-config and update the readme here to point to your new upstream repo. Since you've already fork-released proxy-cfg, this is dubiously useful but if you want the name just let me know.