[#179] sdk/token: Add function to return token issuer

With new neofs-api changes, token issuer will not be stored
in ownerID field of bearer token. We can identify owner by
public key that has been used in signature.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/token/bearer_test.go

@@ -0,0 +1,32 @@
+package token_test
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neofs-api-go/pkg/acl/eacl"
+	"github.com/nspcc-dev/neofs-api-go/pkg/owner"
+	"github.com/nspcc-dev/neofs-api-go/pkg/token"
+	"github.com/nspcc-dev/neofs-crypto/test"
+	"github.com/stretchr/testify/require"
+)
+
+func TestBearerToken_Issuer(t *testing.T) {
+	bearerToken := token.NewBearerToken()
+
+	t.Run("non signed token", func(t *testing.T) {
+		require.Nil(t, bearerToken.Issuer())
+	})
+
+	t.Run("signed token", func(t *testing.T) {
+		key := test.DecodeKey(1)
+
+		wallet, err := owner.NEO3WalletFromPublicKey(&key.PublicKey)
+		require.NoError(t, err)
+
+		ownerID := owner.NewIDFromNeo3Wallet(wallet)
+
+		bearerToken.SetEACLTable(eacl.NewTable())
+		require.NoError(t, bearerToken.SignToken(key))
+		require.Equal(t, bearerToken.Issuer().String(), ownerID.String())
+	})
+}