API: Provide a way to retrieve in bulk UserWithAttributes

The DirectoryManager provides a method searchUsers() which can only return a List of User or String (username). It also provides the methods findUserWithAttributesByName() and findUserWithAttributesByExternalId(), which retrieve one UserWithAttributes.

It would be nice to expand searchUsers() to be able to return UserWithAttributes too so that consumes do not have to first searchUsers(), only to then call findUserWithAttributesByX() n times.