EIP-4844: Implement calc excess data gas

[jangko]

The value of excess_data_gas can be calculated using the parent header and number of blobs in the block.
For the first post-fork block, parent.excess_data_gas is evaluated as 0.

DATA_GAS_PER_BLOB: 2 ** 17
TARGET_DATA_GAS_PER_BLOCK: 2 ** 18

def calc_excess_data_gas(parent: Header, new_blobs: int) -> int:
    consumed_data_gas = new_blobs * DATA_GAS_PER_BLOB
    if parent.excess_data_gas + consumed_data_gas < TARGET_DATA_GAS_PER_BLOCK:
        return 0
    else:
        return parent.excess_data_gas + consumed_data_gas - TARGET_DATA_GAS_PER_BLOCK

This is definetely can be implemented in casper.nim as part of POS consensus utility.
It will be used at txpool and block validation.