Use curl for download on windows when proxy auth is required

[sylvlecl]

This is a proposition to solve the issue reported in this microsoft/vcpkg#38827.

This replaces the first proposed implementation in #1410, following suggestions of @BillyONeal : curl is now used instead of winhttp for download as soon as we have credentials in the URL provided for HTTP proxy (through the HTTPS_PROXY environment variable).

Status:

• Quick implementation to check that it is a better way to go than previous implementation.
  Just checking for "@" in the proxy URL, which might need some refinement ? not obvious though.
• Successfully tested with squid proxy in test env
• Question: should we use curl download as soon as a proxy url is defined ? that would eliminate some code from the winhttp download implementation.

[BillyONeal]

• Question: should we use curl download as soon as a proxy url is defined ? that would eliminate some code from the winhttp download implementation.

I don't think so, there are environments like Server Core that don't come with curl.exe

[BillyONeal]

Thanks!