index block numbers vs hashes

[brenzi]

we currently have no way of querying the block hash of a specific block number.
This, however, is necessary for certain historical queries using the cli with --at <HASH>

[brenzi]

we might workaround theis missing index by using the public subquery of novawallet. but it seems only extrinsics are indexed:
https://explorer.subquery.network/subquery/nova-wallet/nova-wallet-encointer

{
  historyElements(first: 5) {
    nodes {
      blockNumber
      extrinsic
    }
  }
}

[brenzi]

hmm, our rpc supports this:

so we may not even need it and implement this rpc in cli directly

test:

polkadot-js-api rpc.chain.getBlockHash 5555 --ws wss://kusama.api.encointer.org:443