Improve expanding responses

[maksym-tielnyi-solid]

What is done

• Added ExpandableObject that allows expanding fields with multi-level nesting.
  Example of ExpandableObject usage:

  expand: SubscriptionExpandableObject(
    latestInvoice: InvoiceExpandableObject(
      paymentIntent: PaymentIntentExpandableObject(),
      discounts: DiscountExpandableObject(),
    ),
  )

• Added Expandable and ExpandableList classes that can handle both expanded and non-expanded fields. For non-expanded fields Expandable and ExpandableList will contain only IDs and for expanded fields they will additionally contain the expanded value.
• Increased the minimum required SDK version from 2.12.0 to 2.15.0 to use some of the new language features for the implementation.

Notes

• API changes: expandable fields on API models should be wrapped with Expandable<T> which means that the following changes should be done in the existing code:

  // Before
  final latestInvoiceId = subscription.latestInvoice;
  
  // After: we should additionally access the id property of latestInvoice.
  final latestInvoiceId = subscription.latestInvoice.id;
  // Now we can get the whole Invoice object from the subscription object if it was queried.
  final latestInvoice = subscription.latestInvoice?.expanded;