fix: add items to basket with common shipping method

[SGrueber]

PR Type

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no API changes)
[ ] Build-related changes
[ ] CI-related changes
[ ] Documentation content changes
[ ] Application / infrastructure changes
[ ] Other:

What Is the Current Behavior?

The default shipping method is assigned to new basket items. But if there are already items in the basket with a different shipping method the basket will be split into multiple buckets and cannot be ordered by the user any more, because currently multiple buckets are not supported by the pwa.

Issue Number: Closes #

What Is the New Behavior?

The basket common shipping method is assigned to new basket items, so basket items will be added to the existing bucket.

Does this PR Introduce a Breaking Change?

[ ] Yes
[x] No

Steps to repeat:

1. Add a product to cart
2. Start checkout
3. Go to checkout shipping page and change the shipping method.
4. Leave the checkout and add another product to cart.
5. Start checkout and go to review page.
6. -> An error message occurs:

Other Information

AB#85511