Option<B256> for DepositTransaction::source_hash

[Wodann]

The DepositTransaction trait contains a fn source_hash(&self) -> B256; to retrieve the source hash of an OP deposit transaction.

This trait is implemented for the overall OpTransaction, which means that I can also call transaction.source_hash() if transaction is an OpTransaction for e.g. a blob transaction.

This made me wonder whether it wouldn't be better for the API to be:

fn source_hash(&self) -> Option<&B256>

since that would signal whether the transaction has a source hash (only true for deposit transaction).

WDYT, @rakita? If you agree, I can implement this change.

[rakita]

This makes sense, as Transaction for mainnet is flatten, users would expect same thing to happen for Op.

I would completely remove DepositTransaction and add fields to the OpTxTr