Render the payment buttons using ECE behind a feature flag.

[rafaelzaleski]

Implement the rendering of payment buttons using the Express Checkout Element (ECE), controlled by a new feature flag. This involves setting up the feature flag and ensuring the ECE buttons render correctly when the flag is enabled.

Expected behavior:

• With the feature flag enabled, payment buttons render using ECE without issues.
• With the feature flag disabled, payment buttons use the existing rendering method (PRBs).
• It's not expected to be able to process a payment using the ECE implementation.

Tasks

• Create the feature flag
• Implement ECE Payment Buttons using the Stripe ECE documentation. Adjust the button rendering logic to conditionally use ECE based on the feature flag.

[bborman22]

Please add your planning poker estimate with Zenhub @rafaelzaleski