FWS Retrievability Checker

[bajtos]

Build a new FWS service that uses Spark-like nodes to measure the retrievability of data stored on Filecoin.

• The tasking is driven by on-chain data in the SLI smart contract
• Retrievals are implemented as CommP byte-range request (essentially Piece byte-range retrievals)
• The Retrievability Verifier (SLI) contract should be independent of the PDP Verifier (SLI). Others can build bundler services that invoke both verifiers.