Add feature gate WatchListClient for v1.30

Author: tengqm

content/en/docs/reference/command-line-tools-reference/feature-gates/watch-list-client.md

@@ -0,0 +1,18 @@
+---
+title: WatchListClient
+content_type: feature_gate
+_build:
+  list: never
+  render: false
+
+stages:
+  - stage: beta
+    defaultValue: false
+    fromVersion: "1.30"
+---
+This enables an API client (inlcuding some control plane components) to
+request a stream of data rather than an entire list. The behavior change
+is implemented in the client-go library and it is opaque to the client.
+To enable this optimization, you need to enable `WatchList` feature on
+the API server.
+