Workloadspread currently support reschedule pod to other subset if current subset lacks resources, but UnitedDeployment does not. UnitedDeployment should match Workloadspread as far as possbile.

https://openkruise.io/docs/user-manuals/workloadspread/#schedule-strategy

What would you like to be added:
Add schedule-strategy in UnitedDeployment, and support Fixed(current behavior) and Adaptive(missing). When schedule-strategy is adaptive, and the pod in one subset if unschedulable, UnitedDeployment should try next the subset.

Why is this needed:
Enable UnitedDeployment to support use cases such as deployment pods to fixed ECS nodes as many as possible, and if ECS nodes is not enough for new Pods, deploy pods to virtual nodes as serverless pods.