support proxy setting for online installer on CLI

https://doc.qt.io/qtinstallerframework/ifw-use-cases-settings.html
We support proxy on the UI side for the online installer, but on the CLI side, we only support it for the offline installer. I would like to suggest adding a new argument for the proxy setting on the online installer for a better user experience. it is necessary to keep UI and CLI having the same functionality.

  --pu, --proxy-url <url>                        [CLI] Proxy url for server.
                                                 Used only when creating an
                                                 offline installer with site
                                                 license information.