Feature: add sample integration for off-site gateways

[glaubersilva]

Description

This PR adds a Sample Off-Site Gateway Integration which can be used as a starting point (and documentation) for new off-site gateway integrations.

Visuals

Demo Video:
https://www.loom.com/share/12f3cf0338494e03bc9a3170c064b2f1?sid=9cfd3437-aaac-4cfc-9147-ba6362dc3594

Off-site Checkout Page Simulation:

Testing Instructions

1. Pull this branch and then follow these steps: https://github.com/impress-org/givewp-addon-boilerplate?tab=readme-ov-file#setup--installation
2. Go to the Give > Settings > Payment Gateways > Gateways page and enable the sample off-site gateway for both: Visual Form Builder and Option-Based Form Editor;
3. Create a few test donations and make sure everything works as expected.

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

[jonwaldstein]

Thanks!