Manage LDAP group membership through DMX Workspace memberships

Here is an outline of our current plan on how to achieve this:

• Introduce Topic Type "LDAP Group DN" (via declarative Migration)
• Add "LDAP Group DN" Topic Type to to "Workspace" Topic Type (via imperative Migration)
• Monitor postCreateAssoc and postDeleteAssoc
• Create group in LDAP (if group does not exit) identified by "LDAP Group DN"
• Add "membership" DN entry in LDAP for "username"
• Remove "membership" DN entry in LDAP for "username"
• Remove group form LDAP if zero user are member

FYI: @jpn @jri