Add support for proposed tags addition in OCS 1.7

This patch is to add support for the proposed addition of tags to
the OCS standard version 1.7, as seen in https://phabricator.kde.org/T6133

It is supposed to:

• handle the addition of tags to content and download items
• not fail if they are not available (just returning an empty list)

Differential Revision: https://phabricator.kde.org/D6512