feat: Ancillaries Extension

[alldentobias]

Description

This PR introduces an Ancillaries Extension for the Shopping Checkout capability, enabling businesses to offer additional products, services, and insurance tied to line items in a checkout. The extension provides a standardized way for platforms to present upsells, cross-sells, protection plans, and required additions (like recycling fees) while maintaining a clean separation from core checkout fields.

Enhancement Suggestion can be found here

Type of change

• New feature (non-breaking change which adds functionality)
• Documentation update

---

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.