VMState doesn't handle PoA/Clique block header difficulty field

[jangko]

current implementation only handle PoS and PoW difficulty. PoA/Clique difficulty is not handled at all.
a call to DIFFICULTY opcode when executing clique chain will return PoW difficulty instead of Clique block header difficulty.

method difficulty*(vmState: BaseVMState): UInt256 {.base, gcsafe.} =
  if vmState.ttdReached:
    # EIP-4399/EIP-3675
    UInt256.fromBytesBE(vmState.prevRandao.data, allowPadding = false)
  else:
    vmState.chainDB.config.calcDifficulty(vmState.timestamp, vmState.parent)

EVMC handle this situation better as it just copy the difficulty from block header instead of calculating it.