[#577] Add Version() to ExtendedObjectInfo

Signed-off-by: Angira Kekteeva <kira@nspcc.ru>

api/data/tree.go

@@ -9,6 +9,10 @@ import (
 	"github.com/nspcc-dev/neofs-sdk-go/user"
 )
 
+const (
+	UnversionedObjectVersionID = "null"
+)
+
 // NodeVersion represent node from tree service.
 type NodeVersion struct {
 	BaseNodeVersion
@@ -30,6 +34,14 @@ type ExtendedObjectInfo struct {
 	IsLatest    bool
 }
 
+func (e ExtendedObjectInfo) Version() string {
+	if e.NodeVersion.IsUnversioned {
+		return UnversionedObjectVersionID
+	}
+
+	return e.ObjectInfo.ID.EncodeToString()
+}
+
 // BaseNodeVersion is minimal node info from tree service.
 // Basically used for "system" object.
 type BaseNodeVersion struct {