configure
Configures an instance of the Purchases SDK with a specified API key. The instance will be set as a singleton. You should access the singleton instance using Purchases.sharedInstance
Return
An instantiated `Purchases object that has been set as a singleton.
Parameters
The API Key generated for your app from https://app.revenuecat.com/
Optional user identifier. Use this if your app has an account system. If null
`Purchases will generate a unique identifier for the current device and persist it the SharedPreferences. This also affects the behavior of restorePurchases.
Optional boolean set to FALSE by default. Set to TRUE if you are using your own subscription system and you want to use RevenueCat's backend only. If set to TRUE, you should be consuming and acknowledging transactions outside of the Purchases SDK.
Optional ExecutorService to use for the backend calls.