session: change PrivateToken interface methods

This commit replaces PublicKey() and SignData() methods of PrivateToken
with PrivateKey() in order to have the ability to sign data with
session key using service package functions.

session/private_test.go

@@ -1,35 +1,16 @@
 package session
 
 import (
-	"crypto/rand"
 	"testing"
 
-	crypto "github.com/nspcc-dev/neofs-crypto"
 	"github.com/stretchr/testify/require"
 )
 
-func TestPrivateToken(t *testing.T) {
+func TestPToken_PrivateKey(t *testing.T) {
 	// create new private token
 	pToken, err := NewPrivateToken(0)
 	require.NoError(t, err)
-
-	// generate data to sign
-	data := make([]byte, 10)
-	_, err = rand.Read(data)
-	require.NoError(t, err)
-
-	// sign data via private token
-	sig, err := pToken.Sign(data)
-	require.NoError(t, err)
-
-	// check signature
-	require.NoError(t,
-		crypto.Verify(
-			crypto.UnmarshalPublicKey(pToken.PublicKey()),
-			data,
-			sig,
-		),
-	)
+	require.NotNil(t, pToken.PrivateKey())
 }
 
 func TestPToken_Expired(t *testing.T) {