Promo codes that apply discounts based on compare-at-price and target specific customers

Request: Add support to Advanced Discount Code campaigns for promo codes that (1) calculate product discounts from the product.compareatprice (when present) instead of the product's current price, and (2) can be restricted to specific customers. Requirements: - Product discount module: allow selection of discount calculation base: compareatprice (if present) or current price as fallback. - Customer targeting: restrict code usage to a list of customer emails, customer tags, or saved customer segments. - Usage limits: support per-customer limits and total usage limits for the code. - Code generation: support creating a single shared code and bulk-unique codes (upload or generate). - Fallbacks & edge cases: if compareatprice is empty, fall back to regular price; support variant-level compareatprice handling. - UI controls: option to enable/disable stacking with other discounts and to toggle compare-at-price behavior per campaign. - Reporting: surface how the discount was calculated (base price used) in campaign reporting. Notes: Keep compatibility with existing combination rules and schedule options.