Fixes #726

Description

This PR adds support for signup fee coupon during subscription switches. It computes for the actual signup fee amount and applies the discount to that amount.

How to test this PR

1. Go to WooCommerce > Settings > Subscriptions > Switching, and
   • Enable Allow Switching: Between Subscription Variations.
   • For Prorate Recurring Payment: For Upgrades and Downgrades of All Subscription Products
   • For Prorate Signup Fee: Never (charge the full signup fee)
2. Go to Marketing > Coupons and create a coupon: a signup fee % discount for 20%.
3. Create a Variable Subscription product:
   • Under Attributes, add "Membership Tier" with "Regular|Premium" as values, and check Used for variations before saving.
   • Under Variations, create:
     • Regular: 80.00/month, with 5.00 signup fee
     • Premium: 100.00/month, with 5.00 signup fee
4. As a shopper, purchase the "Regular" variation.
5. Go to My Account > Subscriptions > View, and click "Upgrade or Downgrade". Upgrade to "Premium" and go to checkout.
6. Add the signup fee % discount coupon.
7. Verify that the "Order Summary Signup Fee" shows 25.00, but the discount amount is only 1.00 (20% of 5.00, the signup fee prior to upgrade costs).
8. Test for different combinations of Prorate Recurring Payment and Prorate Signup Fee: no matter the combination, the discount amount should never be more than 1.00.
9. Test using different coupon types:
   • signup fee coupon, e.g. 10.00: discount should always be 5.00 or less
   • recurring fee/recurring fee percent coupon: discount should be applied to upgrade cost excluding signup fee.
10. Test a subscription downgrade.

Product impact

• Added changelog entry (or does not apply)
• Will this PR affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• Will this PR affect WooCommerce Payments? yes/no/tbc, add issue ref
• Added deprecated functions, hooks or classes to the spreadsheet