Integrate ECE for Payment Processing in the product page

[rafaelzaleski]

Implement the Stripe Express Checkout Element (ECE) to enable direct payment processing from the product page, bypassing the shopper cart.

Acceptance criteria:

• The ECE must successfully process payments directly from the product page.
• The behavior of express checkout buttons on the product page should mirror that of Payment Request Buttons (PRBs).
• Ensure there are no regressions in Payment Request Buttons (PRB) when ECE is disabled.
• There are no regressions when no express checkout payment methods are active.

[bborman22]

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