LibIMAP: Support for APPEND

Author: https://github.com/X-yl
Commit: 16995dc3d9
Pull-request: https://github.com/SerenityOS/serenity/pull/7735
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/MaxWipfli
Reviewed-by: https://github.com/alimpfard

Userland/Libraries/LibIMAP/Client.h

@@ -34,6 +34,7 @@ public:
     RefPtr<Promise<Optional<ContinueRequest>>> idle();
     RefPtr<Promise<Optional<SolidResponse>>> finish_idle();
     RefPtr<Promise<Optional<SolidResponse>>> status(StringView mailbox, Vector<StatusItemType> const& types);
+    RefPtr<Promise<Optional<SolidResponse>>> append(StringView mailbox, Message&& message, Optional<Vector<String>> flags = {}, Optional<Core::DateTime> date_time = {});
 
     void close();