[#73] Uploader, downloader structures refactoring

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

internal/api/layer/tree_service.go

@@ -0,0 +1,22 @@
+package layer
+
+import (
+	"context"
+	"errors"
+
+	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/api"
+	cid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/container/id"
+)
+
+// TreeService provide interface to interact with tree service using s3 data models.
+type TreeService interface {
+	GetLatestVersion(ctx context.Context, cnrID *cid.ID, objectName string) (*api.NodeVersion, error)
+}
+
+var (
+	// ErrNodeNotFound is returned from Tree service in case of not found error.
+	ErrNodeNotFound = errors.New("not found")
+
+	// ErrNodeAccessDenied is returned from Tree service in case of access denied error.
+	ErrNodeAccessDenied = errors.New("access denied")
+)