feat: Added export finishTransaction() from StoreKit2. (#2664)

Added a previously missing exported function that allows manual
completion of transactions in **StoreKit2**.

Function has already been described in the interface of the
**IosModulePropsSk2** in the codebase but lacked accessible invocation.

The function is crucial and must be used to manually complete
transactions in in-app purchases using **StoreKit2**.

src/modules/iosSk2.ts

@@ -102,3 +102,9 @@ export const beginRefundRequest = (sku: string): Promise<RefundRequestStatus> =>
  */
 export const showManageSubscriptions = (): Promise<null> =>
   RNIapIosSk2.showManageSubscriptions();
+
+/**
+ *
+ */
+export const finishTransaction = (transactionIdentifier: string): Promise<Boolean> =>
+  RNIapIosSk2.finishTransaction(transactionIdentifier);