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 }) => {
  ...
  }