Add external shipping rates app

[rojoangel]

Motivation and Context

Motivation and Context

This change adds the External Shipping Rates sample extension for Adobe Commerce. It is the outcome of the Shipping method extension tutorial on Experience League, which uses the AI coding developer tooling for Adobe Commerce App Builder to generate the extension with the commerce-extensibility toolset (skills and MCP).

The tutorial walks through building a shipping method extension using Adobe App Builder, the Commerce Checkout Starter Kit, and a mock shipping rates API (e.g. Pipedream). This extension adds a configurable shipping method at checkout whose rates come from an external service; merchants set the service URL, API key, and warehouse (ship-from) address in the Admin UI, and the shipping-methods webhook calls the external API and returns options to the customer. The sample is intended for developers learning out-of-process checkout extensibility and as a reference for integrating real shipping rate providers.

How Has This Been Tested?

• Unit tests: npm test (Vitest) in apps/external-shipping-rates.
• Code quality: npm run code:check (Biome) with no reported issues.
• Deploy: aio app deploy (or equivalent) to validate deployment.
• Manual checkout: With a mock API (e.g. Pipedream per the tutorial), configured Service URL and API key in Admin (System → External Shipping Config), and the shipping webhook registered in Commerce Admin; confirmed that mock shipping options (e.g. Mock Standard, Mock Express) appear at checkout.
• Environment: Node 22.x, Adobe I/O CLI, Adobe Commerce 2.4.5+ or Cloud Service (optional to mention).

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.