[#211] blobstor: Define Exists operation

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

pkg/local_object_storage/blobstor/exists.go

@@ -0,0 +1,24 @@
+package blobstor
+
+// ExistsPrm groups the parameters of Exists operation.
+type ExistsPrm struct {
+	address
+}
+
+// ExistsRes groups resulting values of Exists operation.
+type ExistsRes struct {
+	exists bool
+}
+
+// Exists returns the fact that the object is in BLOB storage.
+func (r ExistsRes) Exists() bool {
+	return r.exists
+}
+
+// Exists checks if object is presented in BLOB storage.
+//
+// Returns any error encountered that did not allow
+// to completely check object existence.
+func (b *BlobStor) Exists(prm *ExistsPrm) (*ExistsRes, error) {
+	panic("implement me")
+}