Add support to use custom storage instead of @react-native-async-storage/async-storage(3.0.0-beta2)

[zihejia]

• Add support to use custom storage instead of @react-native-async-storage/async-storage.
  When JavaScript mode is enabled, Mixpanel utilizes AsyncStorage to persist data. If you prefer not to use it, or if AsyncStorage is unavailable in your target environment, you can import or define a different storage class. However, it must follow the same interface as AsyncStorage The following example demonstrates how to use a custom storage solution:

const MyAsyncStorage = require("@my-org/<library-path>/AsyncStorage"); // or your own storage class
const trackAutomaticEvents = false;
const useNative = false;
const mixpanel = new Mixpanel('YOUR_TOKEN', trackAutomaticEvents, useNative, MyAsyncStorage);
mixpanel.init();

• Make optOutTracking and optInTracking consistent with the native SDK.