feat: add support for hgctl gateway-config to retrieve higress gateway config (#274)

Signed-off-by: bitliu <bitliu@tencent.com>

pkg/cmd/hgctl/testdata/config/output/out.route.yaml

@@ -0,0 +1,21 @@
+---
+"@type": type.googleapis.com/envoy.admin.v3.RoutesConfigDump
+dynamic_route_configs:
+- version_info: cb1e51997a9c3aa6f4d920f39fd5bdbd966e9382b7b6bdf42efca8c22c6c3442
+  route_config:
+    "@type": type.googleapis.com/envoy.config.route.v3.RouteConfiguration
+    name: default-higress-http
+    virtual_hosts:
+    - name: default-higress-http
+      domains:
+      - "*"
+      routes:
+      - match:
+          prefix: "/"
+          headers:
+          - name: ":authority"
+            string_match:
+              exact: www.example.com
+        route:
+          cluster: default-backend-rule-0-match-0-www.example.com
+  last_updated: '2023-02-23T09:05:38.448Z'