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.cluster.yaml

@@ -0,0 +1,67 @@
+---
+"@type": type.googleapis.com/envoy.admin.v3.ClustersConfigDump
+version_info: '2'
+static_clusters:
+- cluster:
+    "@type": type.googleapis.com/envoy.config.cluster.v3.Cluster
+    name: xds_cluster
+    type: STRICT_DNS
+    connect_timeout: 1s
+    transport_socket:
+      name: envoy.transport_sockets.tls
+      typed_config:
+        "@type": type.googleapis.com/envoy.extensions.transport_sockets.tls.v3.UpstreamTlsContext
+        common_tls_context:
+          tls_params:
+            tls_maximum_protocol_version: TLSv1_3
+          tls_certificate_sds_secret_configs:
+          - name: xds_certificate
+            sds_config:
+              resource_api_version: V3
+              path_config_source:
+                path: "/sds/xds-certificate.json"
+          validation_context_sds_secret_config:
+            name: xds_trusted_ca
+            sds_config:
+              resource_api_version: V3
+              path_config_source:
+                path: "/sds/xds-trusted-ca.json"
+    load_assignment:
+      cluster_name: xds_cluster
+      endpoints:
+      - lb_endpoints:
+        - endpoint:
+            address:
+              socket_address:
+                address: higress
+                port_value: 18000
+    typed_extension_protocol_options:
+      envoy.extensions.upstreams.http.v3.HttpProtocolOptions:
+        "@type": type.googleapis.com/envoy.extensions.upstreams.http.v3.HttpProtocolOptions
+        explicit_http_config:
+          http2_protocol_options: {}
+  last_updated: '2023-02-23T09:05:23.436Z'
+dynamic_active_clusters:
+- version_info: 2a0a1698a9d3e05b802047b0cd36b52a070afa49042e1ba267168c5265c7cabf
+  cluster:
+    "@type": type.googleapis.com/envoy.config.cluster.v3.Cluster
+    name: default-backend-rule-0-match-0-www.example.com
+    type: STATIC
+    connect_timeout: 5s
+    dns_lookup_family: V4_ONLY
+    outlier_detection: {}
+    common_lb_config:
+      locality_weighted_lb_config: {}
+    load_assignment:
+      cluster_name: default-backend-rule-0-match-0-www.example.com
+      endpoints:
+      - locality: {}
+        lb_endpoints:
+        - endpoint:
+            address:
+              socket_address:
+                address: 0.0.0.0
+                port_value: 3000
+          load_balancing_weight: 1
+        load_balancing_weight: 1
+  last_updated: '2023-02-23T09:05:38.443Z'