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
```

deltachat-rpc-client/tests/test_something.py

@@ -433,7 +433,7 @@ def test_provider_info(rpc) -> None:
     assert provider_info["id"] == "gmail"
 
     # Disable MX record resolution.
-    rpc.set_config(account_id, "socks5_enabled", "1")
+    rpc.set_config(account_id, "proxy_enabled", "1")
     provider_info = rpc.get_provider_info(account_id, "github.com")
     assert provider_info is None