[feature] Better if methods were await/async

[nadjibus]

Thank you for the great library. It would have a lot better if there was no use of events handler. Async/await is a lot better (especially in Forms).

Interface could be:

Task<IEnumerable<InAppProduct>> QueryInventoryAsync(params string[] productsIds) // let the user specify which products he wants to query
Task<PurchaseResult> PurchaseProductAsync(string productId, string payLoad = null) // let the user specify a payload
/// etc...

[simsip-admin]

Thanks Nadjib, great idea!