[#283] pkg/token: Replace SessionToken into a session package

There is a need to add session token to `eacl.Table` structure. To do this,
we need to replace `token.SessionToken` type to another package since `eacl`
package imports `token` one (potential cross-import).

Create `pkg/session` package and replace session token implementation to it.
Related API in `container` package is deprecated from now.

Additionally implement test generator of random session tokens.

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

pkg/session/session_test.go

@@ -0,0 +1,68 @@
+package session_test
+
+import (
+	"crypto/rand"
+	"testing"
+
+	"github.com/nspcc-dev/neofs-api-go/pkg/owner"
+	"github.com/nspcc-dev/neofs-api-go/pkg/session"
+	sessiontest "github.com/nspcc-dev/neofs-api-go/pkg/session/test"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSessionToken_SetID(t *testing.T) {
+	token := session.NewToken()
+
+	id := []byte{1, 2, 3}
+	token.SetID(id)
+
+	require.Equal(t, id, token.ID())
+}
+
+func TestSessionToken_SetOwnerID(t *testing.T) {
+	token := session.NewToken()
+
+	w := new(owner.NEO3Wallet)
+	_, err := rand.Read(w.Bytes())
+	require.NoError(t, err)
+
+	ownerID := owner.NewID()
+	ownerID.SetNeo3Wallet(w)
+
+	token.SetOwnerID(ownerID)
+
+	require.Equal(t, ownerID, token.OwnerID())
+}
+
+func TestSessionToken_SetSessionKey(t *testing.T) {
+	token := session.NewToken()
+
+	key := []byte{1, 2, 3}
+	token.SetSessionKey(key)
+
+	require.Equal(t, key, token.SessionKey())
+}
+
+func TestSessionTokenEncoding(t *testing.T) {
+	tok := sessiontest.Generate()
+
+	t.Run("binary", func(t *testing.T) {
+		data, err := tok.Marshal()
+		require.NoError(t, err)
+
+		tok2 := session.NewToken()
+		require.NoError(t, tok2.Unmarshal(data))
+
+		require.Equal(t, tok, tok2)
+	})
+
+	t.Run("json", func(t *testing.T) {
+		data, err := tok.MarshalJSON()
+		require.NoError(t, err)
+
+		tok2 := session.NewToken()
+		require.NoError(t, tok2.UnmarshalJSON(data))
+
+		require.Equal(t, tok, tok2)
+	})
+}