Added a new endpoint for yearly usage totals using ft_billing.

app/billing/billing_schemas.py

@@ -1,5 +1,6 @@
 from datetime import datetime
 
+
 create_or_update_free_sms_fragment_limit_schema = {
     "$schema": "http://json-schema.org/draft-04/schema#",
     "description": "POST annual billing schema",
@@ -24,3 +25,17 @@ def serialize_ft_billing_remove_emails(data):
         }
         results.append(json_result)
     return results
+
+
+def serialize_ft_billing_yearly_totals(data):
+    yearly_totals = []
+    for total in data:
+        json_result = {
+            "notification_type": total.notification_type,
+            "billing_units": total.billable_units,
+            "rate": float(total.rate),
+            "letter_total": float(total.billable_units * total.rate) if total.notification_type == 'letter' else 0
+        }
+        yearly_totals.append(json_result)
+
+    return yearly_totals