pd control: add service-gc-safepoint command

pd-control.md

@@ -1257,6 +1257,23 @@ scheduler config balance-hot-region-scheduler  // 显示 balance-hot-region 调
     scheduler config balance-hot-region-scheduler set enable-for-tiflash true
     ```
 
+### `service-gc-safepoint`
+
+用于查询当前的 GC safepoint 与 service GC safepoint，输出结果示例如下：
+
+```bash
+{
+  "service_gc_safe_points": [
+    {
+      "service_id": "gc_worker",
+      "expired_at": 9223372036854775807,
+      "safe_point": 439923410637160448
+    }
+  ],
+  "gc_safe_point": 0
+}
+```
+
 ### `store [delete | cancel-delete | label | weight | remove-tombstone | limit ] <store_id> [--jq="<query string>"]`
 
 使用 jq 格式化输出请参考 [jq 格式化 json 输出示例](#jq-格式化-json-输出示例)。