feat(vsfp-61): init function in the middleware

[bartoszherba]

Description

It solves ADR [#004] connection to API

The onCreate function should be called only once, when we start a middleware, then the app should use an instance of creating client for each incoming request - never create a new one. This is an experimental PR that confirms it: https://github.com/vuestorefront/commercetools/pull/190/files - please notice a change in index.server.ts- there is lifted up the state that holds the instance, only that change increases the capacity of our app rapidly.

Types of changes

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

Checklist:

• [x] I have read the CONTRIBUTING document.

Changelog

• [x] I have updated the Changelog (V1) v2 and mentioned all breaking changes in the public API.
• [ ] I have documented all new public APIs and made changes to existing docs mentioning the parts I've changed so they're up to date.

Tests

• [ ] I have written test cases for my code
• [ ] I have tested my Pull Request on production build and (to my knowledge) it works without any issues
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

  I tested manually my code, and it works well with both:

• [ ] Default Theme
• [ ] Capybara Theme

Code standards

• [ ] My code follows the code style of this project.
• [ ] I have followed naming conventions

Docs

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.

[Mateuszpietrusinski]

Hey, we should provide documentation for that functionality.

[lukasborawski]

Yeah, @bartoszherba @filipsobol we're missing docs here.

[bartoszherba]

@filipsobol Could you please double-check the documentation update?

[lukasborawski]

Changelog note is missing as well.

[filipsobol]

❗ Making the createServer async (return a Promise) is a breaking change that should not be a patch, but a minor release. ❗

Please also update the code sample in this section to reflect that createServer is async, eg.:

(async () => {
  const app = await createServer({ integrations });

  app.listen(8181, () => {
    console.log('Middleware started');
  });
})();