LibIMAP: Support for the IDLE command

Author: https://github.com/X-yl
Commit: 1e9dfdcdcc
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

@@ -24,6 +24,8 @@ public:
     RefPtr<Promise<Optional<SolidResponse>>> login(StringView username, StringView password);
     RefPtr<Promise<Optional<SolidResponse>>> list(StringView reference_name, StringView mailbox_name);
     RefPtr<Promise<Optional<SolidResponse>>> select(StringView string);
+    RefPtr<Promise<Optional<ContinueRequest>>> idle();
+    RefPtr<Promise<Optional<SolidResponse>>> finish_idle();
 
     void close();