Add XCM transfer query

[Chralt98]

It's currently complicated to filter the historicalAccountBalances for xcm transfers. I would expect to query historicalXcmTransfers. You can index the TransferredMultiAssets event from here (orml-xtokens pallet). Here is the reference to the subscan information. I would want to know from which account the XCM transfer originated, what the destination account is, what parachain it goes to, how much and which assets are involved etc (the more information the better)..

[saboonikhil]

What do you think about this?

query MyQuery {
  historicalAccountBalances(
    limit: 10
    where: { extrinsic: { name_containsInsensitive: "xtokens" } }
  ) {
    accountId
    assetId
    blockNumber
    dBalance
    event
    timestamp
  }
}