[gateway] set authority header on xDS gRPC bootstrap config

Envoy defaults the :authority header to the cluster name ("xds_cluster")
on gRPC requests. Contour routes by virtual host FQDN, so it returns
HTTP 421 Misdirected Request. Set authority to the xDS server address
so the header matches the expected virtual host.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dilyevsky

pkg/gateway/xds/bootstrap/bootstrap.yaml.tpl

@@ -42,6 +42,7 @@ dynamic_resources:
     grpc_services:
     - envoy_grpc:
         cluster_name: xds_cluster
+        authority: {{ .XdsServer.Address }}
     set_node_on_first_message_only: true
   lds_config:
     ads: {}

pkg/gateway/xds/bootstrap/testdata/render/overload-manager-cgroup.yaml

@@ -22,6 +22,7 @@ dynamic_resources:
     grpc_services:
     - envoy_grpc:
         cluster_name: xds_cluster
+        authority: envoy-gateway
     set_node_on_first_message_only: true
   lds_config:
     ads: {}

pkg/gateway/xds/bootstrap/testdata/render/overload-manager.yaml

@@ -22,6 +22,7 @@ dynamic_resources:
     grpc_services:
     - envoy_grpc:
         cluster_name: xds_cluster
+        authority: envoy-gateway
     set_node_on_first_message_only: true
   lds_config:
     ads: {}