Save shipping to cache storage when property updates

[grimasod]

Related issues

closes #

Short description and why it's useful

The Storage Manager needs to subscribe to the CHECKOUT_UPDATE_PROP_VALUE mutation as well as the CHECKOUT_SAVE_SHIPPING_DETAILS mutation.

Currently, shipping details can become out of synch between the Vuex Checkout store and the Storage Manager cache. When a user completes the Personal Details section and when they select the shipping country, those items are saved to the Vuex store. But those updates are only saved to the cache when the Shipping section is completed.

This PR update the Storage Manager shipping cache when Vuex store is updated.

Screenshots of visual changes before/after (if there are any)

Which environment this relates to

Check your case. In case of any doubts please read about Release Cycle

• Test version (https://test.storefrontcloud.io) - this is a new feature or improvement for Vue Storefront. I've created branch from develop branch and want to merge it back to develop
• RC version (https://next.storefrontcloud.io) - this is a stabilisation fix for Release Candidate of Vue Storefront. I've created branch from release branch and want to merge it back to release
• Stable version (https://demo.storefrontcloud.io) - this is an important fix for current stable version. I've created branch from hotfix or master branch and want to merge it back to hotfix

Upgrade Notes and Changelog

• No upgrade steps required (100% backward compatibility and no breaking changes)
• I've updated the Upgrade notes and Changelog on how to port existing VS sites with this new feature

IMPORTANT NOTICE - Remember to update CHANGELOG.md with description of your change

Contribution and currently important rules acceptance

• I read and followed contribution rules