CAR: Root CID block existence

[darobin]

(I'm moving issues from the CAR spec to here.)

It is unresolved whether an implementation must verify that a CID present in the roots array of the Header also appears as a block in the archive. While it is expected that this would be the case, it is unresolved whether encoders and decoders must validate the existence of root blocks in the archive.

Current implementations of this version of the CAR specification do not check for root block existence in the CAR body.