fix: scale withdrawals amount to gwei

[Nashtare]

Withdrawals amounts that are written in blocks are in gwei unit (cf EIP-4895). We were currently processing them as wei, causing invalid balances on the targets and yielding invalid final state root. This hasn't been caught beforehand because:

• we've tested block contiguity only on John's test chains (which contained no withdrawals)
• benchmarks were targeting single blocks, hence we didn't need to connect the "invalid" final state root with the initial state root of the next block
• we don't check the withdrawals_root in the statement, hence no discrepancy noticed

We'd need to add some sanity checks to compare with the block header, but the response we keep post RPC requests does not currently contain the header's state root. Will add a tracking issue for it.