Add support for DApp Tx streaming from indexer

[uhbif19]

Support for getting all new Tx relevant for DApp from indexer. SDK may hook this with its custom logic and get Txs parsed back into TxSpecs. Storage or any logic is up to SDK user and is out of scope of this task.

Indexer would be Oura or Kupo TBD.

[euonymos]

Closed by #100.