Support groupOfURLs (dynamic groups)

Hello,

I use dynamic groups and it would be interesting to see/manage them trough FD.

This requires to add groupOfURLs objects, like current groups and roles, and to be able to configure memberURL attribute

The member attribute could be visible but not editable.

Note that for autogroup overlay, you also need to add extensibleObject object class, to let the overlay really write the member attribute.