Add: `generateblocks_default_button_attributes` filter

[tomusborne]

This filter allows you to do the following:

add_filter( 'generateblocks_default_button_attributes', function() {
    return [
        'globalClasses' => [ 'gbp-button--primary' ],
    ];
} );

This will make it so every "Button" block you add to your website starts off with the global style, and not any local styles.