feat: shadowsocks support

This change introduces new config options
`proxy_enabled` and `proxy_url`
that replace `socks5_*`.

Tested with deltachat-repl
by starting it with
`cargo run --locked -p deltachat-repl -- deltachat-db` and running
```
> set proxy_enabled 1
> set proxy_url ss://...
> setqr dcaccount:https://chatmail.example.org/new
> configure
```

src/net.rs

@@ -17,6 +17,7 @@ use crate::tools::time;
 
 pub(crate) mod dns;
 pub(crate) mod http;
+pub(crate) mod proxy;
 pub(crate) mod session;
 pub(crate) mod tls;