@VisibleForTesting
  static BaseFeeMarket london(final long londonForkBlockNumber) {
    return london(londonForkBlockNumber, Optional.empty());
  }
Creates a London fee market with the specified fork block number.

@param londonForkBlockNumber the block number at which the London fork activates.
@return a {@link BaseFeeMarket} instance for the London fork.
ethereum/core/src/main/java/org/hyperledger/besu/ethereum/mainnet/feemarket/FeeMarket.java