[#73] Uploader, downloader structures refactoring

Signed-off-by: Marina Biryukova <m.biryukova@yadro.com>

internal/api/tree.go

@@ -0,0 +1,17 @@
+package api
+
+import (
+	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
+)
+
+// NodeVersion represent node from tree service.
+type NodeVersion struct {
+	BaseNodeVersion
+	DeleteMarker bool
+}
+
+// BaseNodeVersion is minimal node info from tree service.
+// Basically used for "system" object.
+type BaseNodeVersion struct {
+	OID oid.ID
+}