[feat] add support for PodDisruptionBudgets for retool backend

[peterwj]

This diff allows users to specify a PodDisruptionBudget in their values file,
instructing Kubernetes to protect the application

Testing plan

1. Create a minikube cluster with two nodes:

$ minikube start
$ minikube node add

2. Install retool into the cluster, with a replica count of three:

$ helm upgrade --install --values values.yaml  myretool . --set replicaCount=3

3. Uncomment the example in the values file, then upgrade again:

$ helm upgrade --values values.yaml  myretool . --set replicaCount=3

4. Confirm presence of PodDisruptionBudget:

$ kubectl describe poddisruptionbudget myretool
Name:             myretool
Namespace:        default
Max unavailable:  1
Selector:         app.kubernetes.io/instance=myretool,app.kubernetes.io/name=retool
Status:
    Allowed disruptions:  0
    Current:              1
    Desired:              2
    Total:                3
Events:                   <none>

5. Attempt to take down a node running two copies of myretool:

± % kubectl drain minikube-m02 --ignore-daemonsets
node/minikube-m02 cordoned
WARNING: ignoring DaemonSet-managed Pods: kube-system/kindnet-zmd4g, kube-system/kube-proxy-g8mhx
evicting pod default/myretool-fcdd777fc-q4hzq
evicting pod default/myretool-fcdd777fc-9b45f
error when evicting pods/"myretool-fcdd777fc-9b45f" -n "default" (will retry after 5s): Cannot evict pod as it would violate the pod's disruption budget.

[kyle-retool]

Thanks!