[ECE] Integrate express payments buttonAttributes API from the Checkout Block

[Mayisha]

Use the buttonAttributes API introduced in Woo Core in woocommerce/woocommerce#47899. This should respect the new height and borderRadius controls on the express checkout block, overriding extension settings when these are available. It should also use the darkMode setting to override the button theme.

This means the Stripe plugin specific settings will still be applied if we're not in a checkout block context. But if we are, the button sizes and colors will be synced across all express payment buttons integrated with this API.

Notes

• Reference WooPayments implementation: Integrate express payments buttonAttributes API from the Checkout Block Automattic/woocommerce-payments#8888
• This is blocked until the buttonAttributes is shipped in the next WooCommerce release (version 9.4.0)