support .spec.updateStrategy.rollingUpdate.maxUnavailable in CRD

[mingmxu]

Describe the current behavior

Currently both BE and FE StatefulSets use the default Maximum unavailable Pods configuration. A rollout could last for long time as it's deployed pod-by-pod.

Describe the enhancement

By exposing the field, application owner could opt to accelerate the rollout. For example it's safe to set 2 when tablet_replica=3. Or it can be set to the same as BE pod replicas to a super-fast rollout.

Additional context

See https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/#maximum-unavailable-pods for the usage.

[mingmxu]

cc @yandongxiao

[yandongxiao]

this feature is LGTM, I think we can support it. @kevincai