Signal holder app when a transaction is done

In the scenario where a holder scans a QR code and an issuer needs a sequence of proof request(s) and/or credential offer(s), the holder app needs a signal to know when the transaction is done so that it can stop waiting, and more importantly resume the process to the caller agent (might be another mobile app)

`goal code` might be helpful to indicate the purpose of scanning a QR Code, but we might need some other message to indicate "done".

## Related Issues
- [x] hyperledger/aries-mobile-agent-react-native#803