feat: Add pod disruption budget to helm chart

Now it is possible to define the pod disruption budget.

Closes: #8222

deploy/charts/emqx/templates/pdb.yaml

@@ -0,0 +1,18 @@
+{{- if and (.Values.pdb.enabled) (.Capabilities.APIVersions.Has "policy/v1/PodDisruptionBudget") }}
+apiVersion: policy/v1
+kind: PodDisruptionBudget
+metadata:
+  name: {{ include "emqx.fullname" . }}-pdb
+  namespace: {{ .Release.Namespace }}
+  labels:
+    app.kubernetes.io/name: {{ include "emqx.name" . }}
+    helm.sh/chart: {{ include "emqx.chart" . }}
+    app.kubernetes.io/instance: {{ .Release.Name }}
+    app.kubernetes.io/managed-by: {{ .Release.Service }}
+spec:
+  maxUnavailable: {{ .Values.pdb.maxUnavailable }}
+  selector:
+    matchLabels:
+      app.kubernetes.io/name: {{ include "emqx.name" . }}
+      app.kubernetes.io/instance: {{ .Release.Name }}
+{{- end }}

deploy/charts/emqx/values.yaml

@@ -211,3 +211,10 @@ ssl:
   issuer:
     name: letsencrypt-dns
     kind: ClusterIssuer
+
+## Setting PodDisruptionBudget.
+## ref: https://kubernetes.io/docs/tasks/run-application/configure-pdb
+##
+pdb:
+  enabled: false
+  maxUnavailable: 1