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-ffi/deltachat.h

@@ -403,11 +403,8 @@ char*           dc_get_blobdir               (const dc_context_t* context);
  * - `send_port`    = SMTP-port, guessed if left out
  * - `send_security`= SMTP-socket, one of @ref DC_SOCKET, defaults to #DC_SOCKET_AUTO
  * - `server_flags` = IMAP-/SMTP-flags as a combination of @ref DC_LP flags, guessed if left out
- * - `socks5_enabled` = SOCKS5 enabled
- * - `socks5_host` = SOCKS5 proxy server host
- * - `socks5_port` = SOCKS5 proxy server port
- * - `socks5_user` = SOCKS5 proxy username
- * - `socks5_password` = SOCKS5 proxy password
+ * - `proxy_enabled` = Proxy enabled. Disabled by default.
+ * - `proxy_url` = Proxy URL. May contain multiple URLs separated by newline, but only the first one is used.
  * - `imap_certificate_checks` = how to check IMAP certificates, one of the @ref DC_CERTCK flags, defaults to #DC_CERTCK_AUTO (0)
  * - `smtp_certificate_checks` = deprecated option, should be set to the same value as `imap_certificate_checks` but ignored by the new core
  * - `displayname`  = Own name to use when sending messages. MUAs are allowed to spread this way e.g. using CC, defaults to empty

deltachat-ffi/src/lib.rs

@@ -4537,19 +4537,16 @@ pub unsafe extern "C" fn dc_provider_new_from_email_with_dns(
     let addr = to_string_lossy(addr);
 
     let ctx = &*context;
-    let socks5_enabled = block_on(async move {
-        ctx.get_config_bool(config::Config::Socks5Enabled)
-            .await
-            .context("Can't get config")
-            .log_err(ctx)
-    });
+    let proxy_enabled = block_on(ctx.get_config_bool(config::Config::ProxyEnabled))
+        .context("Can't get config")
+        .log_err(ctx);
 
-    match socks5_enabled {
-        Ok(socks5_enabled) => {
+    match proxy_enabled {
+        Ok(proxy_enabled) => {
             match block_on(provider::get_provider_info_by_addr(
                 ctx,
                 addr.as_str(),
-                socks5_enabled,
+                proxy_enabled,
             ))
             .log_err(ctx)
             .unwrap_or_default()