Add toJson method for the protocol.

Summary:
To be able to send json as debug protocol, so we can better search and
parse this on akonadiconsole.

Test Plan: build

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: anthonyfieroni, KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D12736