Allow blockstore access to a car directly over the backing storage

[willscott]

Rather than copying / realizing a local carv2 for blockstore access, it would be good to be able to have the blockstore interface with the ReaderAt of the underlying carv1 pointing back to the worker node's unsealed data directly. This will avoid needing to copy the full car, and especially for small/partial retrievals will be important for latency.