Feature/expandable objects

[Andrewangeta]

This PR adds support for expandable objects.

Properties on Stripe objects/entities that can be expanded have been annotated with a new @Expandable property wrapper.

All API routes that can return expanded objects have an extra parameter expand: [String] that allows specifying which objects to expand.
For existing listAll(filter: [String: Any]?) routes on a given resource, you'd just specify the expand key and the array as a value.

Usage:

1. Expanding a single field.

// Expanding a customer from creating a `PaymentIntent`.
stripeclient.paymentIntents.create(amount: 2500, currency: .usd, expand: ["customer"])
.flatMap { paymentIntent in
// Accessing the expanded `StripeCustomer` object   
 paymentIntent.$customer.email
...
}

2. Expanding multiple fields.

// Expanding a customer and payment method from creating a `PaymentIntent`.
stripeclient.paymentIntents.create(amount: 2500, currency: .usd, expand: ["customer", "paymentMethod"])
.flatMap { paymentIntent in
// Accessing the expanded `StripeCustomer` object   
 paymentIntent.$customer?.email // "[email protected]"
// Accessing the expanded `StripePaymentMethod` object
 paymentIntent.$paymentMethod?.card?.last4 // "1234"
 ...
}

3. Expanding nested fields.

// Expanding a payment method and its nested customer from creating a `PaymentIntent`.
stripeclient.paymentIntents.create(amount: 2500, currency: .usd, expand: ["paymentMethod.customer"])
.flatMap { paymentIntent in
// Accessing the expanded `StripePaymentMethod` object
 paymentIntent.$paymentMethod?.card?.last4 // "1234"
// Accessing the nested expanded `StripeCustomer` object   
 paymentIntent.$paymentMethod?.$customer?.email // "[email protected]"
 ...
}

4. Usage with list all.

Note: For list operations expanded fields must start with data

// Expanding a customer from listing all `PaymentIntent`s.
stripeclient.paymentIntents.listAll(filter: ["expand": ["data.customer"...]])
.flatMap { list in
 // Accessing the first `StripePaymentIntent`'s expanded `StripeCustomer` property
  list.data?.first?.$customer?.email // "[email protected]"
}