Enable Lookup of user IDs against a LDAP server

[ieb]

At the moment users are identified by an ID, but there is no more information than that. It would be useful to be able to use a LDAP server to convert the ID into a set of properties associated with the ID. In the case of a userID it might be an eduPerson type structure. In the case of a group there might be a different type of object.

At Cambridge we use IDs called CRSIDs for users that are provided by the authentication mechanism. The LDAP server is visible internally on the Cambridge network to anon lookups, but is not available outside the Cambridge network. If we do this we need to be sure that we are not disseminating information outside the Cambridge network that would not be visible by default.