The default proxy URI that will be used for any URI that doesn't match #GSimpleProxyResolver:ignore-hosts, and doesn't match any of the schemes set with gio.simple_proxy_resolver.SimpleProxyResolver.setUriProxy.
Note that as a special case, if this URI starts with "socks://", #GSimpleProxyResolver will treat it as referring to all three of the socks5, socks4a, and socks4 proxy types.
Builder instance for fluent chaining
Set defaultProxy property.