More Loyalty storefront endpoints for custom rewards pages

Four more endpoints our own Loyalty widgets use are now documented, so a custom rewards page can show the same data.

• No authentication, just your Loyalty GUID: Retrieve public store settings and List currency exchange rates, for converting reward values on multi-currency stores.
• For a logged-in shopper, with the customer token headers: Retrieve the Spotlight view returns their points balance, VIP tier and unused rewards. List free-product rewards returns free-product rewards with each variant's points cost and stock.