IngressRoute: add an option to disable cross-namespace routing

Co-authored-by: Jean-Baptiste Doumenjou <925513+jbdoumenjou@users.noreply.github.com>

pkg/provider/kubernetes/crd/fixtures/with_middleware_cross_namespace.yml

@@ -0,0 +1,58 @@
+apiVersion: traefik.containo.us/v1alpha1
+kind: IngressRoute
+metadata:
+  name: test-crossnamespace.route
+  namespace: default
+
+spec:
+  entryPoints:
+    - foo
+
+  routes:
+  - match: Host(`foo.com`) && PathPrefix(`/bar`)
+    kind: Rule
+    priority: 12
+    services:
+    - name: whoami
+      namespace: default
+      port: 80
+    middlewares:
+    - name: stripprefix
+      namespace: cross-ns
+  - match: Host(`foo.com`) && PathPrefix(`/bir`)
+    kind: Rule
+    priority: 12
+    services:
+    - name: whoami
+      namespace: default
+      port: 80
+    middlewares:
+    - name: test-errorpage
+
+---
+apiVersion: traefik.containo.us/v1alpha1
+kind: Middleware
+metadata:
+  name: stripprefix
+  namespace: cross-ns
+
+spec:
+  stripPrefix:
+    prefixes:
+      - /stripit
+
+---
+apiVersion: traefik.containo.us/v1alpha1
+kind: Middleware
+metadata:
+  name: test-errorpage
+  namespace: default
+spec:
+  errors:
+    status:
+      - 500-599
+    query: /{status}.html
+    service:
+      name: whoami-svc
+      namespace: cross-ns
+      port: 80