feat: add extended api to middleware context

[lsliwaradioluz]

🔗 Linked issue

Quick feature

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation, readme or JSDoc annotations)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality like performance)
• [x] ✨ New feature (a non-breaking change that adds functionality)
• [ ] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Provided easy access to methods added by middleware extensions via the context.extendedApi property.

const extensionA = {
  name: 'extensionA',
  extendApiMethods: {
    methodA: async () => { ... }
  }
}

const extensionB = {
  name: 'extensionB',
  extendApiMethods: {
    methodB: async () => { ... }
  }
}

const extensionC = {
  name: 'extensionC',
  extendApiMethods: {
    methodC: async (context) => {
      context.extendedApi.methodA();
      context.extendedApi.extensionB.methodB();
    }
  }
}

📝 Checklist

• [x] I have linked an issue or discussion.
• [x] I have updated the documentation accordingly.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: ec46f702af7747d04f8d9f0fefed918f5c6f4dd3

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | -------------------------- | ----- | | @vue-storefront/middleware | Minor |

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[lsliwaradioluz]

I will wait with releasing it until we can update the middleware version in Storefront packages