Allow crossprovider service reference

Co-authored-by: Harold Ozouf <harold.ozouf@gmail.com>

pkg/provider/kubernetes/gateway/fixtures/simple_to_api_internal.yml

@@ -0,0 +1,49 @@
+---
+kind: GatewayClass
+apiVersion: networking.x-k8s.io/v1alpha1
+metadata:
+  name: my-gateway-class
+spec:
+  controller: traefik.io/gateway-controller
+
+---
+kind: Gateway
+apiVersion: networking.x-k8s.io/v1alpha1
+metadata:
+  name: my-gateway
+  namespace: default
+spec:
+  gatewayClassName: my-gateway-class
+  listeners:  # Use GatewayClass defaults for listener definition.
+    - protocol: HTTP
+      port: 80
+      routes:
+        kind: HTTPRoute
+        namespaces:
+          from: Same
+        selector:
+          app: foo
+
+---
+kind: HTTPRoute
+apiVersion: networking.x-k8s.io/v1alpha1
+metadata:
+  name: http-app-1
+  namespace: default
+  labels:
+    app: foo
+spec:
+  hostnames:
+    - "foo.com"
+  rules:
+    - matches:
+        - path:
+            type: Exact
+            value: /bar
+      forwardTo:
+        - weight: 1
+          backendRef:
+            group: traefik.containo.us
+            kind: TraefikService
+            name: api@internal
+          port: 80