AppStoreApi.GetSubscriptionAsync() does not support SIMPLE Product.Type

[alvaromarithompson]

SDK you're using (please complete the following information):

• Version 9.0.0

Describe the bug When a subscription on the App Store contains a product whose pricing type is "Simple", retrieving any subscription that contains a product with "SIMPLE" pricing on it throws an exception on deserialization.

To Reproduce Steps to reproduce the behavior:

1. Create a pricing plan that contains "Simple" pricing on it.
2. Subscribe to that plan via the App Store / Testing pages
3. Programmatically call AppStoreApi.GetSubscriptionAsync().
4. An exception is thrown:

Xero.NetStandard.OAuth2.Client.ApiException: 'Error converting value "SIMPLE" to type 'Xero.NetStandard.OAuth2.Model.Appstore.Product+TypeEnum'. Path 'plans[1].subscriptionItems[1].product.type', line 1, position 1536.'

Expected behavior The subscription should be returned containing the relevant subscription item that has product of type SIMPLE

Screenshots App Store pricing type:

Lack of definition of type "SIMPLE" in `Product.TypeEnum":

Additional context The lack of Product.TypeEnum.SIMPLE can be seen at https://github.com/XeroAPI/Xero-NetStandard/blob/18684e235489c2c1acc2770b97e9583b064e788e/Xero.NetStandard.OAuth2/Model/Appstore/Product.cs#L38