Handle PODSI case

[xinaxu]

With PODSI, each deal can include multiple CAR files and has a index to map commd to offset at the final 64MiB

https://github.com/filecoin-project/FIPs/blob/master/FRCs/frc-0058.md#data-segment-index

[xmcai2016]

Including this in RBv2.