Add HTTPS proxy support

[jthomperoo]

The config.go package states that:

geoipupdate/pkg/geoipupdate/config.go

Line 190 in 00434c2

// The http package only supports http and socks5.

I don't think this is accurate, from the Go documentation:
https://pkg.go.dev/net/http#ProxyFromEnvironment

The environment values may be either a complete URL or a "host[:port]", in which case the "http" scheme is assumed. The schemes "http", "https", and "socks5" are supported. An error is returned if the value is a different form.

I believe that HTTPS proxy support should be easy to add based on this, just needing to add https to this if statement:
https://github.com/maxmind/geoipupdate/blob/main/pkg/geoipupdate/config.go#L191