Updates to Storefront Checkout and Server to Server Cart API

• Added amountToPay to Storefront Checkout API

• Exposed email: The same email used for billing address to Storefront Checkout, Storefront Cart and Server to Server Cart API.

• Removed shopper from Storefront Cart. ?include=customer is a replacement.

"customer": {
    "isGuest": boolean,
    "email": "string",
    "firstName": "string",
    "lastName": "string",
    "fullName": "string",
    "addresses": [array]
}

• Added remaining to Storefront Checkout for Gift Certificates. It returns the amount remaining on the gift certificate.

• Added Options to Storefront Cart. This shows product options for physical and digital items.

• Added option_selections to Server to Server Cart API. This provides product options.

• gtin and mpn are now available on products.