POS - Write cart metafield API

[patryk-smc]

Please list the related package(s)

    "@shopify/ui-extensions": "2024.10.0",
    "@shopify/ui-extensions-react": "2024.10.0",

If this related to specific APIs or components, please list them here

POS

Is your feature request related to a problem? Please describe.

In our checkout extensions, we use useApplyMetafieldsChange to add extra data to orders. We aim to maintain a single source of truth for all orders and ensure a consistent process across both checkout and POS. However, the POS only supports addCartProperties, which appears to be limited to cart attributes.

Describe the changes you are looking for

Add an option to write POS cart metafields, similar to checkout.

Describe alternatives you’ve considered

We considered using addCartProperties, but since we pass a significant amount of data in JSON format, it shouldn't be exposed to everyone. Additionally, merchants rely on existing systems that reference our metafield, and they would need to update their integrations. As a public app, this would be quite challenging.

Additional context