[feature request] UnitedDeployment supports capacity planning to subset.

[veophi]

What would you like to be added:

kind: UnitedDeployment
spec:
 replicas: 4
 topology:
   subsets:
   - name:  default-pool
     capacity:
       min: 2
       max: 4
   - name: spot-pool
     capacity:
       min: 0
       max: 2
  - name: elastic-pool
     capacity:
         min: 0

The 4 replicas of UD will be scheduled to default-pool;

If users scale up UD from 4 to 7:

• 2 new replicas will be scheduled to spot-pool;
• 1 new replicas will be scheduled to elastic-pool;

Why is this needed:
Subset capacity planning can be done in advance, and users just update spec.replicas field of UD when scaling up.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.