Implement Outbound proxy feature (Dual-Proxy)

> @Eollone Hi there!
>
> Could you be more specific about the feature? In which cases and why should we use it?
>
> > In some cases, some applications such as adguard vpn that connect to external networks need to go through a specified proxy address.

Assume that a server of Adguard VPN is blocked or limited.(A usual scence in China,Iran or some company.)
Web traffic--->Adguard VPN Client--~~Great~~Firewall-->Adguard VPN Server

And I have my own proxy(or Tor) that can bypass the firewall(or Tor). So I need a **dual-proxy** aka chain proxy.If AdguardVPN can set up an outbound proxy( just like [what Adguard did](https://adguard.com/kb/adguard-for-android/solving-problems/outbound-proxy/) ) it would be more easy to do so.

Web traffic--->Adguard VPN Client----**SOCKS5 or other proxy protocol**----->My Proxy Client--~~Great~~Firewall-->My Proxy Server--->Adguard VPN Server

_Originally posted by @supermagicianck in https://github.com/AdguardTeam/AdGuardVPNForWindows/issues/553#issuecomment-1825890004_