## Description

Implement two separate data loaders -- one for checkpoints by sequence
number, and one for checkpoints by digest.

## Test Plan

Existing tests:

```
sui$ cargo nextest run -p sui-graphql-rpc
sui$ cargo nextest run -p sui-graphql-e2e-tests --features pg_integration
```

And test that the following queries runs much faster now:

```graphql
query {
  transactionBlocks(last: 50) {
    nodes {
      effects {
        checkpoint {
          digest
          sequenceNumber
        }
      }
    }
  }
}
```

```graphql
query {
  c00: checkpoint(id: { digest: "..." }) {
    sequenceNumber
  }
  c01: checkpoint(id: { digest: "..." }) {
    sequenceNumber
  }
  # ...
  c30: checkpoint(id: { digest: "..." }) {
    sequenceNumber
  }
}
```