Migrate from PRB to ECE

[diegocurbelo]

Milestone 0

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1. [ECE] Register domain with new endpoint #3401
   component: ECE +1
   

Milestone 1

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1. [ECE] Integrate express buttons with ECE behind a feature flag #3402
   component: ECE +1
   
2. [ECE] Implement ECE for payment processing on block checkout page #3403
   component: ECE +1
   
3. [ECE] Implement ECE for payment processing on shortcode checkout page #3404
   component: ECE +1
   
4. [ECE] Fix payment method title for orders paid with ECE button #3469
   component: ECE +1
   
5. [ECE] Fix empty space for express checkout container #3470
   component: ECE +1
   

Milestone 2

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1. [ECE] Implement ECE for payment processing on block cart page #3408
   component: ECE +1
   
2. [ECE] Implement ECE for payment processing on shortcode cart page #3409
   component: ECE +1
   
3. [ECE] Implement ECE for payment processing on product page #3410
   component: ECE +1
   
4. [ECE] Implement ECE for payment processing on pay for order page #3411
   component: ECE +1
   
5. [ECE] Spike: Investigate missing parities between ECE and PRB. #3412
   component: ECE +1
   
6. [ECE] Payment processing with coupon and discount #3471
   component: ECE +1
   
7. [ECE] Payment processing with taxes #3472
   component: ECE +1
   
8. Ensure ECE supports basic product purchase #3491
   component: ECE +1
   
9. [ECE] Link method is not displayed at the shortcode checkout #3499
   component: ECE priority: high type: bug +3
   
10. [ECE] Display Stripe Link on the Shortcode cart, Product and Pay for Order pages #3534
    component: ECE +1
    
11. [ECE] Display Stripe Link on the Block cart and checkout pages #3536
    component: ECE +1
    
12. Ensure ECE supports subscriptions #3544
    component: ECE priority: medium type: task +3
    
13. ECE button in settings preview #3578
    component: ECE +1
    

Milestone 3

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1. [ECE] Add unit tests #3405
   component: ECE +1
   
2. [ECE] Set ECE as default express checkout buttons and remove feature flag #3406
   component: ECE +1
   
3. [ECE] Display tax related message #3521
   component: ECE priority: medium type: ux debt +3
   
4. [ECE] Display the line item quantity label in item list #3479
   component: ECE type: enhancement +2
   
5. [GlobalStep] Fatal error "Uncaught Error: Call to a member function get_total() on null" is displayed on "Pages > Add New" page when any "Express checkouts" methods are enabled. #3598
   component: ECE +1
   

Milestone 4

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1. [ECE] Cleanup PRB code and refactor codebase #3407
   component: ECE component: payment request buttons status: blocked +3
2. [ECE] Integrate express payments buttonAttributes API from the Checkout Block #3590
   component: ECE status: blocked +2
3. [ECE] Render the ECE buttons in block editor #3591
   component: ECE +1