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/lib.rs

@@ -84,7 +84,6 @@ mod scheduler;
 pub mod securejoin;
 mod simplify;
 mod smtp;
-mod socks;
 pub mod stock_str;
 mod sync;
 mod timesmearing;