add save_mime_headers to debug info (#5350)

as turned out on recent researches wrt a slow database, setting
`save_mime_headers` will result in 25x larger databases.

this is definetely something we want to know at least in the debug info.

(the option cannot be enabled in current UIs,
however, esp. devs find options to set this.
apart from that, save_mime_headers is only used in some python tests)

ftr: the huge database was more than 10 times slower, leading to missing
notifications on ios (as things do not finish within the few seconds iOS
gave us).
moreover, the hugeness also avoids exporting; this is fixed by
https://github.com/deltachat/deltachat-core-rust/pull/5349

src/context.rs

@@ -804,6 +804,12 @@ impl Context {
             "show_emails",
             self.get_config_int(Config::ShowEmails).await?.to_string(),
         );
+        res.insert(
+            "save_mime_headers",
+            self.get_config_bool(Config::SaveMimeHeaders)
+                .await?
+                .to_string(),
+        );
         res.insert(
             "download_limit",
             self.get_config_int(Config::DownloadLimit)
@@ -1605,7 +1611,6 @@ mod tests {
             "mail_port",
             "mail_security",
             "notify_about_wrong_pw",
-            "save_mime_headers",
             "self_reporting_id",
             "selfstatus",
             "send_server",