[#1255] node/session: Create separate dir for in-memory storage

Move in-memory session storage to the separate directory of `storage`. It is
done for future support of different kind of session storages.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/services/session/storage/temporary/executor.go

@@ -0,0 +1,52 @@
+package temporary
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/google/uuid"
+	"github.com/mr-tron/base58"
+	"github.com/nspcc-dev/neo-go/pkg/crypto/keys"
+	"github.com/nspcc-dev/neofs-api-go/v2/session"
+	"github.com/nspcc-dev/neofs-node/pkg/services/session/storage"
+	"github.com/nspcc-dev/neofs-sdk-go/owner"
+)
+
+func (s *TokenStore) Create(ctx context.Context, body *session.CreateRequestBody) (*session.CreateResponseBody, error) {
+	ownerBytes, err := owner.NewIDFromV2(body.GetOwnerID()).Marshal()
+	if err != nil {
+		panic(err)
+	}
+
+	uid, err := uuid.NewRandom()
+	if err != nil {
+		return nil, fmt.Errorf("could not generate token ID: %w", err)
+	}
+
+	uidBytes, err := uid.MarshalBinary()
+	if err != nil {
+		return nil, fmt.Errorf("could not marshal token ID: %w", err)
+	}
+
+	sk, err := keys.NewPrivateKey()
+	if err != nil {
+		return nil, err
+	}
+
+	privateToken := new(storage.PrivateToken)
+	privateToken.SetSessionKey(&sk.PrivateKey)
+	privateToken.SetExpiredAt(body.GetExpiration())
+
+	s.mtx.Lock()
+	s.tokens[key{
+		tokenID: base58.Encode(uidBytes),
+		ownerID: base58.Encode(ownerBytes),
+	}] = privateToken
+	s.mtx.Unlock()
+
+	res := new(session.CreateResponseBody)
+	res.SetID(uidBytes)
+	res.SetSessionKey(sk.PublicKey().Bytes())
+
+	return res, nil
+}