Get block information for transaction

[SebastienGllmt]

All the watch endpoints all return type TxEvent which has no information about the block the tx is included in

Probably for a lot of use-cases, these need to at least contain some minimal info about the block so that you can query the block later (ex: ChainPoint). This is what Ethereum does

For example, the proposed ReadTxs query (#143) also returns the ChainPoint. If we don't add the ChainPoint to the watch* return types, the flow will end up being

1. Watch for txs and receive cardano.Tx
2. Call ReadTxs on the cardano.Tx received in (1)
3. Call fetchBlock on the ChainPoint received in (2)

It would be nice if I could go directly (1)→(3) without needing step (2), but if having step (2) as an intermediate step is the agreed-upon solution then let's get #143 merged and the process documented

I think something like this is required for #135 anyway