Validates if the given input is mongo id.
Valid values:
validator.mongoId().validate('507f1f77bcf86cd799439011');
validator.mongoId().validate('857g1f77bdf86cd799439011');
Invalid values:
validator.mongoId().validate('507f1f77bcf86cd7994390');
validator.mongoId().validate('507f1f77bcf86cd79943901z');