Move etherscan API keys to each network's config file

[julien51]

Right now we do a bit of magic in the verify task:

  const network = networks[chainId.toString()]

  let etherscanNetworkName
  switch (Number(chainId)) {
    case 1:
      etherscanNetworkName = 'mainnet'
      break
    case 42161:
      etherscanNetworkName = 'arbitrum'
      break
    default:
      etherscanNetworkName = network.name.toLowerCase()
  }
  if (!config.etherscan.apiKey[etherscanNetworkName]) {
    console.error(
      `No Etherscan API key found for '${etherscanNetworkName}'. Please add one to hardhat.config.ts`
    )
    return
  }

and we have to configure etherscan keys in the hardhat config. It would be much better to do that as an etherscan key in each network's config file like this:

etherscan: {
  apiKey: '',
  customChains: {
     //..
  }
}

(of course we would only use customChains if needed.