Fix deserialization logic for polymorphic types

[ob-stripe]

r? @remi-stripe @talljoe-stripe
cc @stripe/api-libraries

Fixes the deserialization logic to properly handle polymorphic types, i.e. properties that are declared with an interface as their type rather than a concrete class.

Summary of changes:

• new Stripe.Infrastructure.Util class that contains:
  • a mapping of object values to concrete types, e.g. "charge" => typeof(Stripe.Charge)
  • a GetConcreteType() method that accepts a "potential type" and an object value:
    • if the potential type is a concrete type (not an interface), return it
    • otherwise, use the object value to find the matching concrete type, and return it (if it's compatible with the interface)
• AbstractStripeObjectConverter, BalanceTransactionSourceConverter, ExternalAccountConverter, PaymentSourceConverter and StripeObjectConverter are all gone
• new StripeObjectConverter deserializer (same name, but completely different logic) that uses Util.GetConcreteType() to figure out which concrete type to instantiate and populate
  • the converter is directly applied to properties with interface types (e.g. EventData.Object), which ensures that the class can be deserialized with JsonConvert.DeserializeObject()
  • the converter is also used as the ItemConverterType for StripeList.Data so that StripeList can be used with interfaces (e.g. StripeList<IPaymentSource> on Customer)
• StringOrObject class used for expandable fields also uses Util.GetConcreteType() to decide which type to pass to JObject.ToObject() for deserialization
• Stripe.Mapper now exposes SerializerSettings property that can be customized by users. Default settings are:
  • Use the StripeObjectConverter deserializer everywhere (which ensures that interfaces can always be deserialized)
  • Don't parse dates (which fixes Metadata containing date is parsed differently when in expanded entity #1178)

Test changes:

• Use JsonConvert.DeserializeObject<> instead of Mapper<>.MapFromJson wherever possible (i.e. when the type is not an interface)
• Add a new wholesome test to ensure that all model classes that implement IHasObject have an entry in the Util.ObjectsToTypes dictionary
• Add a new test to ensure that strings that contain ISO dates are not changed
• Add a test to ensure that serializing/deserializing/reserializing produces consistent results

[remi-stripe]

some minor feedback but that looks great to me.

[ob-stripe]

r? @brandur-stripe

Hey Brandur, mind taking a look at this one? I know you haven't been too involved in stripe-dotnet recently, but I'd like to get as many eyes as possible on this PR. Let me know if you have any questions or need more context.

[brandur-stripe]

Sorry for the delay @ob-stripe! This looks like a huge improvement to me.

I'd be temped to just add a couple basic tests for the new Util class and method, especially for the logic in GetConcreteType at the bottom, but other than that this looks great to me!

[ob-stripe]

I've renamed Util to StripeTypeRegistry and added some tests for GetConcreteType.

Pulling this in!