New webhook topics for SellingPlanGroups

External apps can now subscribe to SellingPlanGroups webhook events to keep track of SellingPlans and SellingPlanGroups lifecycles.

The following are the topics for the events:

• selling_plan_groups/create: Triggered when a new SellingPlanGroup is created.
• selling_plan_groups/update: Triggered when a SellingPlanGroup is updated, including adding/removing/updating SellingPlans.
• selling_plan_groups/delete: Triggered when a new SellingPlanGroup is removed.

To use the topics, an app requires the read_products access scope.