service: Add tablet_operation.hh

A tablet_operation_result struct is added to track the result of a
tablet operation.

service/tablet_operation.hh

@@ -0,0 +1,21 @@
+// Copyright (C) 2024-present ScyllaDB
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+#pragma once
+
+#include <variant>
+#include "gc_clock.hh"
+
+namespace service {
+
+struct tablet_operation_empty_result {
+};
+
+struct tablet_operation_repair_result {
+    gc_clock::time_point repair_time;
+};
+
+using tablet_operation_result = std::variant<tablet_operation_empty_result, tablet_operation_repair_result>;
+
+}
+