Implement new transaction handling available in `cardano-wallet` API

[emesik]

The unstable API is described here:
https://input-output-hk.github.io/cardano-wallet/api/edge/#tag/Transactions-New

Our top-level API could consist of:

• Wallet.transfer() accepts brodacast=True argument, which preserves current behavior by default
• Transaction.body can hold the serialized transaction, and call to Wallet.transfer(..., broadcast=False) always returns such objects
• Wallet.sign(tx: Transaction) -> Transaction will accept only those with body and return signed ones
• Wallet.submit(tx: Transaction) -> None will attempt to submit the transaction and raise exceptions on failure

Questions:

1. How to broadcast signed transaction? There's Submit External Transaction method which even mentions regular submitTransaction in the doc, however the latter is not present anywhere right now.
2. What is the balance method?