Implement Stripe Express Checkout Elements (ECE) on the Product page

[mattallan]

Fixes #3410

Important

This PR is based off the changes in #3436. This branch will need to be updated to include any future changes made to that base branch.

Changes proposed in this Pull Request:

This PR implements Stripe's ECE (Google Pay & Apple Pay buttons) on the product page:

Chrome	Safari

Testing instructions

There are A LOT scenarios to cover when testing these buttons from the product page. Let's start with the basics:

1. Enable the ECE feature flag in the wp-options table.

update_option( '_wcstripe_feature_ece', 'yes' );

2. Make sure Apple Pay and Google Pay are enabled:
   
3. Create a simple WC Product and visit the front-facing Product page
4. Change the quantity of the product, confirm the ECE button was blocked and unblocked
5. Click on the Google Pay button
6. Confirm the order details and totals in Google Pay are correct
7. Change shipping methods between free and flat rate and confirm price is correct

Free	Flat rate

8. Complete the google pay purchase
9. Confirm the order is created in WooCommerce
10. Confirm the charge in Stripe is correct
    

---

• 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)

[mattallan]

Full checklist I'm testing

• Simple product
• Variable product
• Set quantity to 2 or 3 (confirm element is blocked and unblocked)
• Bookings product
• Simple subscription
• Variable subscription
• Synced subscription with no initial payment
• Simple product with deposit
• Variable product with deposit
• Simple subscription with deposit
• Variable subscription with deposit
• Virtual/downloadable product
• Virtual subscription
• Store without taxes
• Store with taxes
  • Prices include tax - tax applies
  • Prices include tax - outside of taxable region
  • Prices exclusive of tax

Non-successful test cases

• Error notices when using a declining card
• Abort/cancel ECE window and re-open
• Use an invalid shipping address

[wjrosa]

Hey Matt! I see the changelog.txt entry is missing 👀

[wjrosa]

Awesome work, Matt!

4. Change the quantity of the product, confirm the ECE button was blocked and unblocked

(tried to get a blocked image, but the loading is too fast)

6. Confirm the order details and totals in Google Pay are correct

7. Change shipping methods between free and flat rate and confirm price is correct

9. Confirm the order is created in WooCommerce

10. Confirm the charge in Stripe is correct

It does not work on my Android phone, but none of our previous implementations for this project does. We need to double check later.

[mattallan]

Thanks @wjrosa for the review!
I've pushed up fea0f12 to add the changelog entry back in