New planHandle field for managed pricing app subscription plans

App subscription plans created with managed pricing now include plan handle data. This makes it easier and more consistent to query for app subscription plans, because these human-readable handles persist even if the plan ID changes due to updates or edits.

planHandle is available on the AppRecurringPricing object, while plan_handle is included in the app_subscriptions/update webhook topic.

With this update, you'll now be asked to define your own plan handle when creating new app subscription plans through your Partner Dashboard. All existing plans have been updated with a planHandle value based on their plan name.