Webhooks mechanism for IdoSell Apps

To ensure that third-party developers can use the Admin Webhook API, the mechanism has been mapped to the IdoSell Apps panel. The developer can define webhook parameters (URL, headers, events, etc.) in the app add/edit form, under the “Technical data” tab. The webhooks are then automatically configured in the stores, after the merchant purchases an app subscription, and removed when the merchant unsubscribes. Like app API keys, webhooks added via IdoSell Apps are not visible in the merchant's panel.