Finish Transaction iOS

[brettbatie]

Adds the ability to call finishTransaction for an iOS transaction, addressing issue #1.

An example of using finish transaction is the following:

const chargeResult :any = await BillingPlugin.launchBillingFlow({product: 'basic_monthly_10', type: 'subscription'});

// Call the backend to verify the order

await BillingPlugin.finishTransaction({transactionId: chargeResult.storeKitTransactionID});

[carsten-klaffke]

Hey, thanks a lot for adding this. I tested your code and it looks all good! I wasn't aware of that "finish"-method. Looks cleaner now. 👍