The default proxy URI that will be used for any URI that doesn’t
GSimpleProxyResolver:ignore-hosts, and doesn’t match any
of the schemes set with g_simple_proxy_resolver_set_uri_proxy().
Note that as a special case, if this URI starts with
GSimpleProxyResolver will treat it as referring
to all three of the socks5, socks4a, and socks4 proxy types.