Email <=> XMPP gateway

Libervia is a versatile communication ecosystem offering features like instant messaging, blogging, event planning, photo albums, file sharing, audio/video calls, and more. It can additionally function as an XMPP component, providing server-side features. This initiative focuses on creating an Email <=> XMPP gateway, enhancing file management for attachments, transforming mailing list threads into interactive, forum-style discussions with modern elements such as tags and mentions, and ensuring support for end-to-end encryption. The Libervia interface will also see improvements for a better user experience, with clear indicators of message origins and security status. This gateway is a move toward unifying various communication methods within single clients, following Libervia's philosophy as seen with its ActivityPub <=> XMPP gateway and is in harmony with other projects like Slidge, Spectrum 2, or Biboumi. With the introduction of this component, not only will Libervia's functionality be elevated, but it will also equip other XMPP ecosystem projects with the ability to connect their users with the email world, fostering deeper integration of XMPP across the spectrum of communication tools.