content-discovery: query interface

[Gozala]

Once we have #743 and storacha/w3infra#188 we should have a way to write and index claims we issue regarding subject content partition (which CARs is it in) and location (where those CARs can be fetched from).

To make it possible for others to use this information we would want to expose a query interface through which clients could figure out URLs and byte ranges they can fetch necessary data from to re-assemble the content.

Task here is to design & implement such a query interface which would be a think wrapper over actual index in dynamo.