Support optional owner_type in UI extension metafields

[kumar303]

WHY are these changes introduced?

For checkout and customer account UI extensions, we're introducing owner_type as an optional field for metafield declarations. See UI-TAG spec (private link).

WHAT is this pull request doing?

This enables TOML configuration like:

[[extensions.metafields]]
namespace = "$app"
key = "energy-rating"
owner_type = "PRODUCTVARIANT"

[[extensions.metafields]]
namespace = "custom"
key = "age-verification-required"
owner_type = "CUSTOMER"

The owner_type allows surfaces to optimize metafield fetching in ways that was previously not possible.

API change to support this https://meteorite.shopify.io/repos/shop/world/pulls/2062315/files (private)

How to manually test your changes?

TODO

Post-release steps

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add