cpp: Expose gpgme_decrypt_result_t.is_mime through cpp API

* lang/cpp/src/descriptionresult.cpp (DescriptionResult::isMime): New.
* lang/cpp/src/descriptionresult.h: Update accordingly.

This exposes the is_mime metadata from a decryption result to users
of the C++ library.

• GnuPG-bug-id: T6199
• Signed-off-by: Carl Schwan <carl.schwan@gnupg.com>