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/Subscriptions.php

@@ -27,7 +27,8 @@ class Subscriptions extends BaseCollection
      *      ends_at: null|string,
      *      trial_days?: null|int,
      *      trial_ends_at?: null|string,
-     *      trial_once?: bool
+     *      trial_once?: bool,
+     *      discount?: null|array{label: string, percent_off?: float|string, amount_off?: float|string, duration: string, duration_in_periods?: int}
      *  } $attributes The subscription data:
      *    - group:          The group (optional)
      *    - name:           The name
@@ -42,6 +43,7 @@ class Subscriptions extends BaseCollection
      *    - trial_days:     A free trial of 1–365 days from the user's confirmation on (optional)
      *    - trial_ends_at:  Or a fixed end of the free trial (ISO 8601, in the future; optional)
      *    - trial_once:     Refuse the trial if the user already had one with the app in the group (optional)
+     *    - discount:       A discount per billed period, see AppSubscriptions::create() (optional)
      * @throws Throwable
      */
     public function create(array $attributes): Subscription