fix: duplicate watcher id

[lhlxc]

Use watcher list length as watcher id could lead to duplicate watcher id, then the exist watcher will lost. This could happen when some watcher stopped and new watcher create.

1. create two watcher.
   id: 0
   id: 1

2. watcher with id 0 stopped，watcher list length is 1 now.

higress-standalone/src/apiserver/pkg/registry/file_rest.go

Lines 689 to 693 in 1d58038

	func (w *fileWatch) Stop() {
	w.f.fileWatchersMutex.Lock()
	delete(w.f.fileWatchers, w.id)
	w.f.fileWatchersMutex.Unlock()
	}

3. create new watcher, it's id is the watcher list length 1, which is duplicate with existing watcher.

higress-standalone/src/apiserver/pkg/registry/file_rest.go

Lines 644 to 649 in 1d58038

	func (f *fileREST) Watch(ctx context.Context, options *metainternalversion.ListOptions) (watch.Interface, error) {
	fw := &fileWatch{
	id: len(f.fileWatchers),
	f: f,
	ch: make(chan watch.Event, 10),
	}

[CH3CHO]

Good catch. Thanks.