Refactor: add support to the gateway webhook events API

[glaubersilva]

Depends on impress-org/givewp#7664

Description

This PR refactors the OffSiteGateway sample to use the new gateway webhook events API to handle webhook notifications.

For better context check this PR description: impress-org/givewp#7664

Affects

The OffSiteGateway sample.

Visuals

Testing Instructions

1. Pull this branch;
2. Create a few test donations and on the Off-site Checkout Page Simulation click on the Send Webhook Notification To Change Donation Status To Complete ⭷ link;
3. The Webhook Notification Handler page should be displayed and after clicking on the link there you should see the event for the webhook notification test created in the Action Scheduler list table.

Pre-review Checklist

• Acceptance criteria satisfied and marked in related issue
• Relevant @unreleased tags included in DocBlocks
• Includes unit tests
• Reviewed by the designer (if follows a design)
• Self Review of code and UX completed