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
```

node/test/test.mjs

@@ -271,7 +271,7 @@ describe('Basic offline Tests', function () {
       'sync_msgs',
       'sentbox_watch',
       'show_emails',
-      'socks5_enabled',
+      'proxy_enabled',
       'sqlite_version',
       'uptime',
       'used_account_settings',