As a swatch developer, I want to simplify the calculation of tally values for billingCategory, so that I can avoid edge cases with retallies that make is impossible to provide a useful value.

First, applicable contract capacity should be fetched via the customer-facing capacity endpoint (/api/rhsm-subscriptions/v1/capacity/products/{product_id}/{metric_id} endpoint). This will require generation of a client to call the capacity API from TallyResource. (Implemented this way to avoid unnecessary coupling of tally and capacity API implementations). The client should re-use the x-rh-identity used to call TallyResource endpoints.

When billingCategory is prepaid, the value should be (pseudo-code):

min(snapshotValue, contractCapacity) 

When billingCategory is on-demand, the value should be (pseudo-code):

max(snapshotValue - contractCapacity, 0)

QE automation work tracked in SWatch-1526