feat: Ancillaries Extension

[alldentobias]

EDIT: Had to recreate the PR here to adhere to the CLA: #235

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

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Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[google-cla]

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