Verifiable Retrievals

[bajtos]

Provide end-to-end verifiability for Filecoin retrievals.

First, we need to work with FilOz and Curio to enhance the piece-retrieval (PDP/CommP-retrieval) endpoint to return inclusion proof for all byte-range requests.

Second, we should rework Spark to perform piece-byte-range retrievals (instead, or in addition, to the current IPFS-payload-block retrievals) and verify the inclusion proof returned by the server.