Add proxy support in kvm

- Add proxy option to "kvm upgrade"
- "kvm upgrade" uses global proxy (http_proxy env var) by default
- Support authenticated proxies

parent aspnet/KRuntime#215
Not super sure if you need the `[ref]` here. If it works without it, might be cleaner to remove it. Also in theory you could do
``Add-Proxy-If-Specified $wc`` (without the round brackets) and it should work. Not particularly important though.