A finance application stores invoice documents in an Azure Cosmos DB for NoSQL container. Each invoice has a 'subtotal' numeric property but does not persist the tax amount, because the regional tax rate varies and is supplied at query time. Analysts frequently run queries that must return each invoice's subtotal plus a calculated tax value using a rate they pass in. The team wants a reusable, server-side calculation that can be referenced inside SELECT and WHERE clauses of ad-hoc SQL queries without modifying stored documents. Which server-side feature should they implement?