Display progress for pending completion cells

[robatwilliams]

While waiting for a long completion, it may be useful to have some feedback on the progress of the API call.

The built-in Excel behaviour is to show #BUSY! and a refresh icon in the cell while the formula (API request) is pending.

Perhaps this could somehow be extended to display progress by using streaming. On a per-cell basis, this would make the cell value unstable while streaming, leading to wave after wave of cascading changes to other cells - there'd need to be a way to prevent that. Doing something in the sidebar would mean we start having our own UI.