Fork DAGStore and extended `ShardAccessor` to return an `io.Reader`

[Wondertan]

According to #1099, the DAGStore.ShardAccessor has to return an io.Reader. Unfortunately, it does not do so, and we should fork the DAGStore repo and provide such functionality there.

Additionally, we should make issues in the upstream repo to discuss it with the original implementors and potentially submit a fix there to avoid maintenance of the fork.

[distractedm1nd]

I didn't see you were assigned when I did this, sorry, just changed assignee to me if thats fine

[Wondertan]

Totally fine

[distractedm1nd]

@Wondertan Closing because celestiaorg/dagstore#1 is merged