[enhancement] built-in http and socks proxy support

[momobobe]

It would be fantastic to add support for built-in http and socks proxy, like many F-Droid clients (e.g. Neo Store). System VPN is one thing, but sometimes a more flexible and specific proxy is required (like tor). So please consider to add this feature.

P.S. It's different from Github proxy or so. This should be a proxy for all connections that the App makes.