docs: End-to-end private routing

[dettanym]

For integration: refer to the Go LibP2P Kad DHT v2-develop branch

• It's being tested and will be eventually merged into main. Monitor this pull request --- you can see the files changed in the pull request.
• DHT type v2/dht.go: It includes a routing table field named rt and a DHT co-ordinator named kad.
• Instantiating Routing Table interface (with non-generics): v2/coord/routing/probe.go. A field of this type is used in the DHT interface.
• DHT co-ordinator v2/coord/coordinator.go. A field of this type is used in the DHT interface. Its Query function is also called on the client-side.
• Messages: Can introduce privateFindPeer and privateFindProviders messages here.

Server-side: v2/handlers.go:

1. handleFindPeer and handleGetProviders as in v1. Both take requests as the arguments, which include the key. Hopefully, through Normalization #8 we would've developed private versions of the requests. These methods call closerPeers which is also implemented in the same file.
2. closerPeers: It internally calls d.rt.NearestNodes which again, we would've implemented privately (without the key).

Client-side:

1. v2/routing.go: Implements FindPeer and FindProvidersAsync methods. (Sidenote: interfaces includes these two methods are defined in the Go LibP2P core repo.) Both methods run d.kad.Query with the key as an argument and a function fn to check whether the node in the last hop has the correct CID.
2. DHT co-ordinator Query method: It looks up its own RT for the closest nodes and then currently, sends in a fake PB message with the key. So we need a private implementation of the Query method which does not send the key in the PB message (PIR request).