Make sample message rule more relevant based on entity

[mahalakshme]

Case:
Currently in sample message rule independent of for which entity type(Individual/Encounter/Enrolment) it is for, it has the below line: const individual = params.entity;

Instead if it is for encounter, the code should have been const encounter = params.entity; like we have already done in visit schedule rule and decision rule.

Need to fix:
For self-implementing organisations and implementation team, this will help to understand better how to access the entity.