Allow to read a contextData object on DocumentModelJsonReader

For instance, the current Edit layout in the Web UI does not display a field to set the change comment, as it was possible in the default Edit form in the JSF UI.

This comment was bound to a field in the contextData of the DocumentModel

        <field>contextData['request/comment']</field>

We need to find a way to have a similar feature.

Note that the contextData question may be more global as the context data map is also used to disable some listeners, so we may want to keep this feature.

We should be able to read a contextData object when posting/putting a JSON document on the REST API.
Improve the DocumentModelJsonReader to read it.