chore: add auto backup workspace setting

proto/store/workspace_setting.proto

@@ -0,0 +1,30 @@
+syntax = "proto3";
+
+package slash.store;
+
+option go_package = "gen/store";
+
+message WorkspaceSetting {
+  WorkspaceSettingKey key = 1;
+
+  oneof value {
+    AutoBackupWorkspaceSetting auto_backup = 2;
+  }
+}
+
+enum WorkspaceSettingKey {
+  WORKSPACE_SETTING_KEY_UNSPECIFIED = 0;
+
+  WORKSPACE_SETTING_AUTO_BACKUP = 1;
+}
+
+message AutoBackupWorkspaceSetting {
+  // Whether auto backup is enabled.
+  bool enabled = 1;
+  // The cron expression for auto backup.
+  // For example, "0 0 0 * * *" means backup at 00:00:00 every day.
+  // See https://en.wikipedia.org/wiki/Cron for more details.
+  string cron_expression = 2;
+  // The maximum number of backups to keep.
+  int32 max_keep = 3;
+}