Improvement: allow any parameters for checkout()

[chrisvdm]

• Create object for checkout mutation (StripeCheckoutParamsInput) in schema

 gql`
      mutation Checkout(
        $params: StripeCheckoutParamsInput
      ) {
        checkout(params: $params) {
          id
          url
        }
      }
    `

• use ...restParams in hooks and services

async ({ cart, customer, successUrl, cancelUrl, mode, allowPromotionCodes, ...restParams }) => {
...
}