refactor: move plugins to contrib dir (#1399)

* move to contrib dir


Co-authored-by: chenzhihui <zhihui_chen@foxmail.com>

contrib/registry/zookeeper/watcher.go

@@ -0,0 +1,41 @@
+package zookeeper
+
+import (
+	"context"
+
+	"github.com/go-kratos/kratos/v2/registry"
+)
+
+var (
+	_ registry.Watcher = &watcher{}
+)
+
+type watcher struct {
+	ctx    context.Context
+	cancel context.CancelFunc
+	event  chan struct{}
+	set    *serviceSet
+}
+
+func (w watcher) Next() (services []*registry.ServiceInstance, err error) {
+	select {
+	case <-w.ctx.Done():
+		err = w.ctx.Err()
+	case <-w.event:
+	}
+	ss, ok := w.set.services.Load().([]*registry.ServiceInstance)
+	if ok {
+		for _, s := range ss {
+			services = append(services, s)
+		}
+	}
+	return
+}
+
+func (w *watcher) Stop() error {
+	w.cancel()
+	w.set.lock.Lock()
+	defer w.set.lock.Unlock()
+	delete(w.set.watcher, w)
+	return nil
+}