Add actual rolled up weight computation

What does this MR do and why?

Computes rolled up weight based on descendant issues to have feature parity with legacy epics

Related to #463219 (closed)

MR acceptance checklist

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How to set up and validate locally

Task weights are not included for now so that we don't have to handle cases where a work item (issue) has a set weight and also has a rolled up weight value. Related to #474687 (closed)