Read epic data from the WorkItem in the REST API

What does this MR do and why?

Read epic data from the WorkItem in the REST API

This changes the way how we read data for epics in the REST API, but reading directly from the work item, instead of the epic object.

Since the work item is the source of truth as most services write to the work item first, we should start reading from it in our APIs.

This does not braek the API because the data is bi-directionally synced.

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Part of &17316

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