Correctly implement the coinbase reward calculation

[tizoc]

Current implementation is a workaround because the coinbase_iter method iterates over the coinbase fees, not the coinbase transactions:

https://github.com/openmina/openmina/blob/5c88393478373b4e09ad80553f91cfe39cd9190e/mina-p2p-messages/src/v2/manual.rs#L1072-L1076

From the actual coinbase transaction the amount field can be used to obtain the real amount.