Add /data/{id}/tree endpoint to fetch trees of type DATA_TREE

[bhufmann]

The provider type DATA_TREE already exists. However, the endpoint has not been added.

Existing client applications are using the XY or timegraph equivalent tree endpoint instead. This will add the dedicated endpoint for data trees.

Signed-off-by: Bernd Hufmann bernd.hufmann@ericsson.com