[SMS App] Export all addresses of multitarget messages

Summary

Export the complete list of remote addresses of a multitarget message

Note that this changes format of the returned Message object, replacing the string "address" field with a string list "addresses" field, so it is not backwards-compatible with old desktop applications

Test Plan

See Test Plan of the desktop-side patch: https://invent.kde.org/kde/kdeconnect-kde/merge_requests/101