Discount subscriptions per billed period

discount (label, percent_off or amount_off, duration once, repeating or
forever) when creating subscriptions and per subscription of checkout
sessions, the discount field on subscriptions and the
subscription.discount_ended event type.

src/Id/Resources/AppSubscription.php

@@ -28,6 +28,7 @@ use JsonSerializable;
  * @property ?string $status draft, trial, active, paused, cancelling, expired or cancelled, see status()
  * @property ?string $cancels_at when a scheduled cancellation takes effect
  * @property ?string $trial_ends_at
+ * @property ?object $discount {label, percent_off, amount_off, duration, duration_in_periods, periods_remaining}; periods_remaining is null for `forever`, 0 once it ended
  * @property ?string $starts_at
  * @property ?string $ends_at
  * @property ?string $accounting_at