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
Just tested and seems I don't need `[ref]` here. They are removed in latest commit.
I keep the round brackets for consistency with all other invocations in this file. But thank you for the hint, it's good for people new to PowerShell script like me :smile: