democratic-csi: Enable volume snapshot support

The _democratic-csi_ controller can create Synology LUN snapshots based
on VolumeSnapshot resources.  This feature can be used to e.g. create
data snapshots before upgrades, etc.

democratic-csi/democratic-csi.yaml

@@ -270,6 +270,38 @@ spec:
             fieldRef:
               apiVersion: v1
               fieldPath: metadata.name
+      # https://github.com/kubernetes-csi/external-snapshotter
+      # beware upgrading version:
+      #  - https://github.com/rook/rook/issues/4178
+      #  - https://github.com/kubernetes-csi/external-snapshotter/issues/147#issuecomment-513664310
+      - name: external-snapshotter
+        image: "registry.k8s.io/sig-storage/csi-snapshotter:v8.2.1"
+        args:
+        - --v=5
+        - --leader-election
+        - --leader-election-namespace=democratic-csi
+        - --timeout=90s
+        - --worker-threads=10
+        - --csi-address=/csi-data/csi.sock
+        volumeMounts:
+        - mountPath: /csi-data
+          name: socket-dir
+        env:
+        - name: NODE_NAME
+          valueFrom:
+            fieldRef:
+              apiVersion: v1
+              fieldPath: spec.nodeName
+        - name: NAMESPACE
+          valueFrom:
+            fieldRef:
+              apiVersion: v1
+              fieldPath: metadata.namespace
+        - name: POD_NAME
+          valueFrom:
+            fieldRef:
+              apiVersion: v1
+              fieldPath: metadata.name
       - name: csi-driver
         image: docker.io/democraticcsi/democratic-csi:latest
         args: