Change relay arg from block number to block rlp

[k06a]

Allows to use with any network. We serialise web3js header to rlp with this code:

const blockFromRpc = require('ethereumjs-block/from-rpc')
const utils = require('ethereumjs-util');

function web3BlockToRlp(blockData) {
    blockData.difficulty = parseInt(blockData.difficulty, 10);
    blockData.totalDifficulty = parseInt(blockData.totalDifficulty, 10);
    blockData.uncleHash = blockData.sha3Uncles;
    blockData.coinbase = blockData.miner;
    blockData.transactionTrie = blockData.transactionsRoot;
    blockData.receiptTrie = blockData.receiptsRoot;
    blockData.bloom = blockData.logsBloom;
    const blockHeader = blockFromRpc(blockData);
    return utils.rlp.encode(blockHeader.header.raw);
}

const blockRlpHex = web3.utils.bytesToHex(web3BlockToRlp(await web3.eth.getBlock(blockNumber)));
const proof = JSON.parse(await execute(`./ethashproof/cmd/relayer/relayer ${blockRlpHex} | sed -e '1,/Json output/d'`));

[tranvictor]

Working with the block itself instead of the block number is a good idea, however, I think we better make another command line for that instead of changing this one.

[tranvictor]

@k06a can you take a look at my comment and change accordingly?