Support new transaction format

In the scope of INDY-1123, we've been changing the format of transactions:
see https://github.com/hyperledger/indy-node/blob/master/docs/transactions-new.md#common-structure

We need to support it in SDK in a way that both old and new formats are supported.
This is needed to be able to use SDK in tests on Node.

Please note that format of Requests is not changed.