[#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/services/object/internal/key.go

@@ -0,0 +1,19 @@
+package internal
+
+import (
+	"bytes"
+
+	"github.com/nspcc-dev/neofs-api-go/v2/session"
+	"github.com/nspcc-dev/neofs-node/pkg/core/client"
+)
+
+// VerifyResponseKeyV2 checks if response is signed with expected key. Returns client.ErrWrongPublicKey if not.
+func VerifyResponseKeyV2(expectedKey []byte, resp interface {
+	GetVerificationHeader() *session.ResponseVerificationHeader
+}) error {
+	if !bytes.Equal(resp.GetVerificationHeader().GetBodySignature().GetKey(), expectedKey) {
+		return client.ErrWrongPublicKey
+	}
+
+	return nil
+}