public void set_proxy_resolver (ProxyResolver? proxy_resolver)
Overrides the ProxyResolver used by this.
You can call this if you want to use specific proxies, rather than using the system default proxy settings.
Note that whether or not the proxy resolver is actually used depends on the setting of enable_proxy, which is not changed by this function (but which is true by default)
a ProxyResolver, or null for the default.