Add support for ECE Payment buttons on Pay for Order page

[james-allan]

Closes #3411

🛑 This PR is based on the WIP changes in #3436. That PR should be resolved before merging this one.

Changes proposed in this Pull Request:

This PR adds support for processing Express payment button payments via the Pay for Order page.

^{Google Pay and Apple Pay buttons on the Pay for Order}

Testing instructions

SETUP

1. Enable Apple Pay / Google Pay in your Stripe plugin settings.
2. Enable the ECE feature flag.
   • Update the _wcstripe_feature_ece option. See WC_Stripe_Feature_Flags::is_stripe_ece_enabled()

PAYMENTS

1. Go to WooCommerce → Orders on your WP Admin dashboard.
2. Click the Add order button.
3. Add your current user as the customer.
4. Add any items.
5. Calculate totals.
6. Create the order.
7. In the order details section click the "Customer payment page" link. (Screenshot)
8. On the following page note that the Apple Pay and/or Google Pay buttons are shown.
9. Choose either Google Pay or Apple Pay
10. Confirm the modal that pops up has correct totals etc.

Order totals	Apple Pay screens

13. Complete the payment.
14. Confirm you are redirected to the order received page.
15. In your Stripe Dashboard confirm the payment is completed and the total matches the order total.

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)