[#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/core/client/util.go

@@ -1,8 +1,10 @@
 package client
 
 import (
+	"bytes"
 	"fmt"
 
+	"github.com/nspcc-dev/neofs-api-go/pkg/client"
 	"github.com/nspcc-dev/neofs-node/pkg/network"
 )
 
@@ -40,3 +42,15 @@ func NodeInfoFromNetmapElement(dst *NodeInfo, info interface {
 }) {
 	nodeInfoFromKeyAddr(dst, info.PublicKey(), info.Addresses())
 }
+
+// AssertKeyResponseCallback returns client response callback which checks if the response was signed by expected key.
+// Returns ErrWrongPublicKey in case of key mismatch.
+func AssertKeyResponseCallback(expectedKey []byte) func(client.ResponseMetaInfo) error {
+	return func(info client.ResponseMetaInfo) error {
+		if !bytes.Equal(info.ResponderKey(), expectedKey) {
+			return ErrWrongPublicKey
+		}
+
+		return nil
+	}
+}