Add ability to track subdirectories create/delete/rename event for file-provider

[shawnh2]

• we should be handling the various change events in this region of code:
  

  gateway/internal/provider/file/file.go

  Lines 151 to 159 in d611d33

  	switch event.Op {
  	case fsnotify.Create, fsnotify.Write, fsnotify.Remove:
  	// Since we do not watch any events in the subdirectories, any events involving files
  	// modifications in current directory will trigger the event handling.
  	goto handle
  	default:
  	// do nothing
  	continue
  	}

• we should add tests for cases where the user creates, deletes, or renames a complete directory in one of the watched directories

Originally posted by @EshaanAgg in #5126 (comment)