Changes to List Subscriptions

added the required property items/items/customer/first_user_event_at to the response with the 200 status

added the new optional query request parameter started_after

added the new optional query request parameter started_before

added the required property items/items/discount/anyOf[subschema #1: SubscriptionDiscount]/oneOf[subschema #1: DiscountFixedOnceForeverDurationBase]/max_redemptions_per_customer to the response with the 200 status

added the required property items/items/discount/anyOf[subschema #1: SubscriptionDiscount]/oneOf[subschema #2: DiscountFixedRepeatDurationBase]/max_redemptions_per_customer to the response with the 200 status

added the required property items/items/discount/anyOf[subschema #1: SubscriptionDiscount]/oneOf[subschema #3: DiscountPercentageOnceForeverDurationBase]/max_redemptions_per_customer to the response with the 200 status

added the required property items/items/discount/anyOf[subschema #1: SubscriptionDiscount]/oneOf[subschema #4: DiscountPercentageRepeatDurationBase]/max_redemptions_per_customer to the response with the 200 status