feat(coredns): Add PDB for coredns

In order to keep coredns always available, we deploy a PDB, just in case
:)

infrastructure/coredns/kustomization.yaml

+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+namespace: kube-system
+resources:
+  - poddisruptionbudget.yaml

infrastructure/coredns/poddisruptionbudget.yaml

+apiVersion: policy/v1
+kind: PodDisruptionBudget
+metadata:
+  name: coredns
+  namespace: kube-system
+spec:
+  minAvailable: 1
+  selector:
+    matchLabels:
+      k8s-app: kube-dns

infrastructure/kustomization.yaml

 apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 resources:
+  - coredns
   - kyverno
   - firewall
   - metallb