Improvement: allow additional props

[chrisvdm]

Problem: Need a way to pass Stripe additional properties like when items need to be expanded
Solution: update sdl to accept a data param that accepts the "payload" and additional properties for Stripe
example

retrieveStripeCustomer(customerID, {expand: ['subscriptions']})

translates to

variables: {
  data: {
    id: customerID,
    addProps: {expand: ['subscriptions']}
  }
}

or something like that

*This does not apply to listing functions like listStripeSubscriptions. Additional properties are passed in the "payload"