[Desktop] Make message syncronization interface capable of handling future changes

Summary:
Add "event" field to ConversationMessage

Update packet type with proper field names and commenting

Future "proof" SmsPlugin against new event types

Test Plan:

• Install the corresponding Android-side patch D16600
• Verify that it is possible to synchronize messages the same as it was before

Reviewers: KDE Connect, apol

Reviewed By: KDE Connect, apol

Subscribers: apol, kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D16599