Add priorityClassName in PropagationPolicy & ClusterPropagationPolicy

[seanlaii]

What type of PR is this?
/kind api-change
/kind feature

What this PR does / why we need it:
Add SchedulePriority to PropagationPolicy & ClusterPropagationPolicy to support flexible priority class resolution strategies:

• KubePriorityClass: Use Kubernetes native PriorityClass
• PodPriorityClass: Use priority from workload's PodTemplate
• FederatedPriorityClass: Reserved for future Karmada-specific priority classes

This enhancement allows users to:

1. Specify which priority class source to use
2. Control resource scheduling priority
3. Maintain consistent priority behavior with Kubernetes

Which issue(s) this PR fixes:
Part of #5961

Special notes for your reviewer:

• Default source is KubePriorityClass
• Falls back to global default if unspecified
• FederatedPriorityClass is reserved for future implementation
• All new fields are optional to maintain backward compatibility

Does this PR introduce a user-facing change?:

Add SchedulePriority field to PropagationPolicy and ClusterPropagationPolicy:
- Supports multiple priority class sources (Kubernetes, PodTemplate, Federated[future])
- Configurable priority class resolution strategy
- Maintains compatibility with Kubernetes priority behavior
- Optional fields preserve backward compatibility

[karmada-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign kevin-wangzefeng for approval. For more information see the Kubernetes Code Review Process.

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• OWNERS
• pkg/apis/OWNERS

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.33%. Comparing base (5bfdf61) to head (25e0f40).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5962   +/-   ##
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  Coverage   48.33%   48.33%           
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  Files         666      666           
  Lines       54858    54858           
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+ Hits        26516    26518    +2     
+ Misses      26617    26616    -1     
+ Partials     1725     1724    -1     

Flag	Coverage Δ
unittests	48.33% <ø> (+<0.01%)	⬆️

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