Add migration for pure proxies from the AH migration for Polkadot and Kusama

[Tbaut]

I went the lazy way. Computing the list of existing pure proxies e.g with

query GetPolkadotPureProxiesBeforeBlock {
  proxyAccounts(
    where: {
      creationBlockNumber_lt: 30423691
      delegator: {
        isPureProxy_eq: true
        id_startsWith: "kusama-"
      }
    }
    orderBy: creationBlockNumber_ASC
  ) {
    id
    delegator {
      id
      pubKey
      isPureProxy
    }
    delegatee {
      id
      pubKey
    }
    type
    delay
    createdAt
    creationBlockNumber
    extrinsicIndex
  }
}

And then, at a specific block, where I know there's a multisig event happening (so it'll be indexed) I've applied the migration and created all the pure proxies, for each chain.