Add the possibillity to specify a proxy.
The proxy can be set anytime, even before the registration (important for countries where tg has been blocked), see: https://github.com/tdlib/td/issues/300
Documentation is here: https://core.telegram.org/tdlib/docs/classtd_1_1td__api_1_1set_proxy.html
We should add a setting with a button on the registration screen and in the settings.