List subscription items

Adds an item to a running subscription, or replaces one item with another when you send `remove_item_id` (an upgrade or downgrade). The item is a product (`product_id`) or a custom item (`name`, `type`, `reference_price` and `reference_currency`, no `product_id`). Answers `201` with the new item when the change applies now, or `200` with the pending item when it is scheduled for the end of the current period (then nothing is charged and the item shows in `add_at_current_period_end` or the replaced item's `change_at_current_period_end`). Money can move. When the change applies at once, the customer pays the prorated amount for the rest of the current period now (from their account balance first, then their saved payment method) and an invoice is made; a cheaper replacement or lower quantity is refunded prorated, to the original payment method where possible, otherwise to the account balance. Whether a change applies at once or at the period end depends on the immediate upgrade and downgrade product types in your subscription settings, or on `apply_immediately`. On a live subscription only one money-moving change is allowed every 10 minutes. Use the preview endpoint first to show the customer the amount. Secret, trusted and limited tokens may call it. A limited token only for the customer it is signed in as; it may not send prices, discounts or `apply_immediately`, may only replace product items, and only when your subscription settings allow customer upgrades or downgrades.