Add "free_allowance_units" to service usage APIs

This represents the number of chargeable_units that were actually
free due to the free allowance - they won't be included in "cost".

Although the existing calculations in Admin [^1][^2] will still be
correct with a change in SMS rates - it's cost that's the problem
- it makes sense to have all the knowledge about calculating usage
consistently in these two APIs.

Note that the Integer casting is covered by the API-level tests in
test_rest.

[^1]: 474d7dfda8/app/main/views/dashboard.py (L490)
[^2]: c63660d56d/app/main/views/dashboard.py (L350)

app/billing/billing_schemas.py

@@ -24,6 +24,7 @@ def serialize_ft_billing_remove_emails(rows):
             "rate": float(row.rate),
             "postage": row.postage,
             "cost": float(row.cost),
+            "free_allowance_used": row.free_allowance_used,
         }
         for row in rows
         if row.notification_type != 'email'
@@ -42,6 +43,7 @@ def serialize_ft_billing_yearly_totals(rows):
             # TEMPORARY: while we migrate to "cost" in the Admin app
             "letter_total": float(row.billable_units * row.rate) if row.notification_type == 'letter' else 0,
             "cost": float(row.cost),
+            "free_allowance_used": row.free_allowance_used,
         }
         for row in rows
     ]