static BaseFeeMarket fixedBaseFee(final long londonForkBlockNumber, final Wei fixedBaseFee) {
    return new FixedBaseFeeMarket(londonForkBlockNumber, fixedBaseFee);
  }
Creates a fee market with a fixed base fee.

@param londonForkBlockNumber the block number at which the London fork activates.
@param fixedBaseFee the fixed base fee to use.
@return a {@link BaseFeeMarket} instance with a fixed base fee.
ethereum/core/src/main/java/org/hyperledger/besu/ethereum/mainnet/feemarket/FeeMarket.java