Updates an existing inventory item's variants by product ID.
The updateInventoryVariantFieldsByProductId() function returns a Promise that resolves when the
variant with the specified product ID has been updated.
Only the properties passed in the inventory item object will be updated. All other properties will remain the same.
Note: You can also update a variant's inventory by inventory ID instead of product ID using the updateInventoryVariantFields() function.
The corresponding product ID for the inventory item in a Wix store.
The information to update in the variant.