Add Gift Card Expiration Config and Webhook Name

The schema now includes a new enum GiftCardConfigurationExpirationUnit with values DAYS, MONTHS, and YEARS. An object type GiftCardExpirationConfiguration exposes two non‑null fields: expirationUnit and expirationValue. GiftCardConfiguration is extended with a nullable field expirationConfiguration that returns a GiftCardExpirationConfiguration if the shop has set a default expiration. The GiftCardConfiguration structure is unchanged otherwise, enabling reading or updating the default expiration settings through the new field. WebhookSubscription receives a new string field name, providing a human‑readable identifier for each subscription. Existing queries, mutations, and directives remain unchanged; only these added fields affect data retrieval and mutation logic.