[Tasks Assignment] User system integration: Ability to read user and roles from external source

Define/implement any required methods for getting users information. The planning server must be able to get the following information:

1) list of existing users
2) with the corresponding groups
3) any additional information that could be useful for working with concepts like skills/affinities.

But this information is currently not exposed by the kie-server APIs in fact all the tasks assignments operations realized by the jBPM runtime are produced by using the following API:
UserGroupCallback:
boolean existsUser(String userId);
boolean existsGroup(String groupId);
List<String> getGroupsForUser(String userId);

Phase 1 will only consider WF defined users, other user repositories that can be plugged into the jBPM runtime like LDAP are not in scope.