feat(accessor): exposing ShardAccessor.data through exported method

[distractedm1nd]

Ref: celestiaorg/celestia-node#1101

@Wondertan Is this what you had in mind? Or should it be a ReadCloser, or the mount.Reader?

[Wondertan]

@distractedm1nd, we just need to ensure that all the resources that are allocated for the Reader are cleaned up once we have done the reading.

Note that we return the Reader and ShardAccessor.Close is not accessible from there, while it must be closed once the reader is done with the job.

Another solution here would be to make ShardAccesor itself to implement Reader and it already returns Close