Watched Directory Watch Modify

Added :ref:watch_modify <envoy_v3_api_field_config.core.v3.WatchedDirectory.watch_modify> field to :ref:WatchedDirectory <envoy_v3_api_msg_config.core.v3.WatchedDirectory>. When set to true, the watcher subscribes to Modified (IN_MODIFY) inotify events in addition to MovedTo (IN_MOVED_TO). This allows in-place file writes to trigger reload callbacks, enabling secret managers that write certificate files directly (rather than via atomic rename) to trigger SDS certificate rotation. By default, only move/rename events are watched.