feat: better cart and order item attributes, with labels

[aldeed]

Resolves #5208
Impact: minor
Type: feature

Issue

CartItem and OrderItem have attributes array, but label was null and value was pulled from the wrong product field. OrderItem always had empty array.

One underlying problem is that we had no label for variants, which is why we have null attribute labels.

A related problem is general confusion around title vs. optionTitle fields for variants.

Solution

• Added optional attributeLabel string field to Products of type "variant"
• Added required attributeLabel string field to all CatalogProductVariant
• Added an operator UI field to the variant edit form, "Attribute label". Also generally improved the labels, placeholder text, and help text for the attributeLabel, optionTitle, and title fields on that form, to help avoid more confusion.
• Properly set attributes array on OrderItems when they are created
• Properly set attributes array on CartItems when they are created
• Migration to set attributeLabel to a default value in Products and Catalog collections. For top-level variants, set to "Variant". For option variants, set to "Option".
• Migration to set label for all attributes on all CartItem to a default value in Cart collection. For first attribute, set to "Variant". For second attribute, if present, set to "Option".

NOTE: I opted not to do #5206 in this PR to avoid it becoming too large.

Breaking changes

As long as migrations run, nothing should break. If client code is using cart item attributes and depending on what the value of them currently is (the full title), it may need to be updated.

Testing

1. Test the new and updated fields at the top of the variant edit form. Test for top-level variant and for option variant. Verify you cannot publish without an attribute label set.
2. Verify you can query GraphQL for attributeLabel field for any variant or option in a CatalogItemProduct.
3. Test the migration and verify that all data is migrated as described.

[mikemurray]

👍

[willopez]

👍