BZ1158818 Add "reminder" feature

you should not rely on concrete implementation of the model directly. Use TaskModelProvider (https://github.com/droolsjbpm/droolsjbpm-knowledge/blob/master/kie-internal/src/main/java/org/kie/internal/task/api/TaskModelProvider.java) and its factory (https://github.com/droolsjbpm/droolsjbpm-knowledge/blob/master/kie-internal/src/main/java/org/kie/internal/task/api/TaskModelFactory.java) to get hold of proper implementation available on runtime. See example here https://github.com/droolsjbpm/jbpm/blob/master/jbpm-human-task/jbpm-human-task-core/src/test/java/org/jbpm/services/task/LifeCycleBaseTest.java#L1137