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/session.rs

@@ -1,3 +1,4 @@
+use crate::net::proxy::ShadowsocksStream;
 use async_native_tls::TlsStream;
 use fast_socks5::client::Socks5Stream;
 use std::pin::Pin;
@@ -44,6 +45,11 @@ impl<T: SessionStream> SessionStream for Socks5Stream<T> {
         self.get_socket_mut().set_read_timeout(timeout)
     }
 }
+impl<T: SessionStream> SessionStream for ShadowsocksStream<T> {
+    fn set_read_timeout(&mut self, timeout: Option<Duration>) {
+        self.stream.get_mut().set_read_timeout(timeout)
+    }
+}
 
 /// Session stream with a read buffer.
 pub(crate) trait SessionBufStream: SessionStream + AsyncBufRead {}