[Sequester] Refactor extensions for issue => work item transformations

[BillWagner]

As Sequester has grown features, there are several extension methods on the GH issue / pull request types that compute various fields for the associated work item.

These methods should be refactored into a single extension method that returns a struct (or record) that computes all those items.

It may also fix the record used for story points per iteration. That's a bit ugly as well.

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Associated WorkItem - 320273