Split by custom expression: add the expression to compare variables

[mahajantejas]

refer the discord thread https://discord.com/channels/717975833226248303/1427878378668818503

<%= case "@results.assigned_arm_id" do "@results.control_arm_id" -> "Control"; "@results.treatment_arm_id" -> "Treatment"; end %>

add the details to enable NGOs to use similar expressions in the flow

[mahajantejas]

#535