Two new storefront configs

Today we added two new design configs for the storefront:

• Control expandable billing section at the checkout: https://api-docs.ecwid.com/reference/checkout-pages#control-expandable-billing-address-section-on-the-checkout
• Set storefront language for search engine crawlers for better SEO of multilingual website: https://api-docs.ecwid.com/reference/customize-behaviour#set-storefront-language-for-search-engine-crawlers