Add extra Capacity Units (CUs) to F SKU - at reservation price

[frithjof_v]

We'd like the ability to add CUs to a capacity.

Say we have a workspace that needs 90 CUs.

Today, we'd need to buy an F128 for this workspace.

Instead, we'd like to buy an F64 and buy 26 extra CUs - all 90 CUs at reservation price.