[META] Support Shopify Buy Button

Problem/Motivation

The current integration with Shopify relies on the poorly documented and marginally supported return_to request parameter. It appears that this functionality is no longer completely supported by Shopify, since redirects no longer reliably return users to the originating page, see #2903116: Make "return_to" configurable.

Proposed resolution

Shopify's recommended way of integrating with external sites is the buy button: https://help.shopify.com/en/manual/online-sales-channels/buy-button. This collection of issues will introduce buy button support with the ultimate goal of removing the return_to-based behavior.

Remaining tasks

• #3188495: Add Buy Button styling configuration options
• #3188499: Add Buy Button layout configuration options
• #3188501: Add Buy Button cart labeling configuration options
• #3188505: Add Buy Button cart styling configuration options
• #3188517: Add storefront access token field configuration
• #3188518: Remove product options column
• #3188753: Create buy button element on product pages
• Issue for UI help text improvements
• #3189389: Update cart block implementation to support buy button
• #3191097: Render buy button elements outside of iframe
• #3191098: Allow configuration of buy button library version
• #3191101: Add configuration options for displaying variant-specific data
• #3194249: Add configurable checkout behavior
• Issue to remove ShopifyProductVariant entity
• #3195251: Add test coverage for buy button integration