Add username information in the new tag service implementation

With the current tag service, it is possible to retrieve which user tagged a document. This allows for example a user to untag a document if he was the one who tagged it in the first place, even with only Read access to this document.
To intregrate it in the new tag service implementation, the data model needs to be rediscussed to add this information (as in the first implementation done by NXP-21651, it consists in an array of labels stored in a facet).