[Deferred intent] Card - Implement saving cards in the checkout and using saved ones

[a-danae]

Acceptance criteria

• The behavior for users must continue to be the same as with the legacy PE implementation
• Allowing saving cards and used saved ones must continue to follow the "Enable payments via saved cards" setting
• Checking off the "save card" checkbox does save the card in:
  • The block checkout
  • The shortcode checkout
  • The Pay for order page
• Saved cards show up on:
  • The block checkout
  • The shortcode checkout
  • The Pay for order page
  • The My account -> Payment methods page
• When using a saved payment method, its related billing address must be updated with the billing address specified in the order
• Deleting a saved card under the My account -> Payment methods page removes the card from the WC store and from the Stripe customer. We likely don't need changes here, but let's confirm it
• Errors must be handled gracefully:
  • A relevant, user-friendly message must be shown to shoppers
  • A relevant note must be added to the order
  • A detailed message of the error must be logged to the log files if enabled
  • The order status must reflect the payment status
• The following critical flows and e2e tests must behave as expected (use the legacy PE implementation as a reference if in doubt):
  • Shopper > Save card at checkout
  • Shopper > Checkout with saved card
  • Shopper > Delete payment method
  • Shopper > (Blocks) Save card at checkout
  • Shopper > (Blocks) Checkout with saved card

Out of scope

• Adding new cards under the My account -> Payment methods page
• Adding 3DS cards

[a-danae]

This is working as expected. The e2e tests won't fail at this point because the legacy code hasn't been removed. Let's tackle those separately to include other flows too.