Add support for additional classes for the add to cart button

[senadir]

Describe the bug

WooCommerce added support for additional classes in the add to cart button 15 months go, this is still not reflected in the Woo Subs add to cart custom buttons, causing a broken style.

<button type="submit" name="add-to-cart" value="<?php echo esc_attr( $product->get_id() ); ?>" class="single_add_to_cart_button button alt<?php echo esc_attr( wc_wp_theme_get_element_class_name( 'button' ) ? ' ' . wc_wp_theme_get_element_class_name( 'button' ) : '' ); ?>"><?php echo esc_html( $product->single_add_to_cart_text() ); ?></button>

To Reproduce

1. In 2024 theme, go to a subscription product page, see the button is without styles.
2. Go to a regular product page, see that the button has regular styles.

Expected behavior

Subscription button should match the regular button styles.

Actual behavior

Button is without styles.

Product impact

• [x] Does this issue affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• [x] Does this issue affect WooCommerce Payments? yes/no/tbc, add issue ref