[#645] client/cache: Check response public key in all client operations

There is a need to check if public key in the RPC response matches the
public key of the related storage node declared in network map.

Define `ErrWrongPublicKey` error. Implement RPC response handler's
constructor `AssertKeyResponseCallback` which checks public key. Construct
handler and pass it to client's option `WithResponseInfoHandler`.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/network/cache/client.go

@@ -62,7 +62,9 @@ func (c *ClientCache) Get(info clientcore.NodeInfo) (client.Client, error) {
 		return cli, nil
 	}
 
-	cli := newMultiClient(netAddr, c.opts)
+	cli := newMultiClient(netAddr, append(c.opts,
+		client.WithResponseInfoHandler(clientcore.AssertKeyResponseCallback(info.PublicKey())),
+	))
 
 	c.clients[cacheKey] = cli