Add demo for vertical federated learning (#9103)

demo/nvflare/config/config_fed_client.json

@@ -1,23 +0,0 @@
-{
-  "format_version": 2,
-  "executors": [
-    {
-      "tasks": [
-        "train"
-      ],
-      "executor": {
-        "path": "trainer.XGBoostTrainer",
-        "args": {
-          "server_address": "localhost:9091",
-          "world_size": 2,
-          "server_cert_path": "server-cert.pem",
-          "client_key_path": "client-key.pem",
-          "client_cert_path": "client-cert.pem",
-          "use_gpus": "false"
-        }
-      }
-    }
-  ],
-  "task_result_filters": [],
-  "task_data_filters": []
-}