Implement scheduler `GetProviders`

[gpestana]

GetProviders is the beef of the anonymous scheduler. Its goal is to order a peer to ask for data blocks in a way that provides plausible deniability to the peers (e.g. a peer want content A, but it is requested by the schedulers to fetch and replicate chunks of content A from different peers and content B and C too).

The main goal of the GetProviders is to come up with a replication manifest which will allow the peer which requested a certain resource to issue multiple network requests which will not only obtain all blocks of the requested resource, but also doing it in a plausible deniable way, i.e. non of the network peers will be able to "allege" that the peer is explicitly requesting the resource.