Evaluate network wide retrievability check

[pyropy]

We need to add a new Spark score that measures how many deals (CIDs) can be retrieved from the network using any available retrieval provider, including non-Filecoin nodes running IPFS.

This new score is useful as a network-wide metric only. It must not affect the current per-miner/per-client/per-allocator RSR metrics. We shouldn't even collect it with per-miner/per-client/per-allocator granularity.

Blocks CheckerNetwork/spark-stats#360