Implement of "Protected Headers for Cryptographic E-mail". T742

Summary:
Currently protected headers are in draft status for an RFC:
https://datatracker.ietf.org/doc/draft-autocrypt-lamps-protected-headers/

Test Plan: run autotests successfully

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D28448