add dispatch workflow to deploy load balancer

As the Kubernetes load balancer service for the mongo-express application
does not require redeployment on every application change, add a dispatch
workflow to perform the deployment of the load balancer service only
when triggered by an administrative dispatch event.

This GitHub Actions workflow expects an Azure Kubernetes Service cluster
to exist and for its name and resource group to be configured as
GitHub Actions secrets, as well as the Azure login credentials.

After deployment of the persistent volumes manifest, the workflow
uses kubectl to wait for the service's ingress IP to be assigned.
Unfortunately this can not be done exclusively with the "kubectl wait"
command as its JSON condition matching requires a fixed string, and
the IP address cannot be known in advance.  Instead, use "kubectl get"
in a loop and wait for the load balancer's status to contain an
"ingress" key, as suggested in:

kubernetes/kubernetes#80828 (comment)

See also this concern regarding how JSON condition matching was
implemented for "kubectl wait":

kubernetes/kubernetes#83094 (comment)

.github/workflows/deploy-app-service.yml

@@ -0,0 +1,39 @@
+name: Deploy Application Load Balancer
+
+on:
+  workflow_dispatch:
+
+jobs:
+  deploy:
+    name: Load Balancer Service Deployment
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout Git branch
+        uses: actions/checkout@v3
+
+      - name: Login to Azure
+        uses: azure/login@v1
+        with:
+          creds: ${{ secrets.AZURE_CREDENTIALS }}
+
+      - name: Set Azure Kubernetes Service configuration
+        uses: azure/aks-set-context@v3
+        with:
+          resource-group: ${{ secrets.RESOURCE_GROUP }}
+          cluster-name: ${{ secrets.CLUSTER_NAME }}
+
+      - name: Deploy application load balancer service
+        uses: Azure/k8s-deploy@v4
+        with:
+          action: deploy
+          manifests: |
+            manifests/app-service.yml
+
+      - name: Setup kubectl
+        uses: azure/setup-kubectl@v3
+
+      - name: Wait for service ingress IP assignment
+        run: |
+          timeout 300s sh \
+            -c 'until kubectl get service frontend --output=jsonpath='{.status.loadBalancer}' | grep ingress; do : ; done'