Bugfix: WorkloadSpread cannot patch priorityClassName

[AiRanthem]

Ⅰ. Describe what this PR does

1. Bugfix: WorkloadSpread cannot patch priorityClassName

Ⅱ. Does this pull request fix one issue?

fixes #1607

Ⅲ. Describe how to verify it

E2E and UT are added. You can verify manually with a yaml like:

apiVersion: apps.kruise.io/v1alpha1
kind: WorkloadSpread
metadata:
  name: workloadspread-demo
spec:
  targetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: sleep
  subsets:
    - name: subset-1
      patch:
        spec:
          priorityClassName: high-priority
      maxReplicas: 1
    - name: subset-2

Ⅳ. Special notes for reviews

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.94%. Comparing base (0d0031a) to head (71690f5).
Report is 142 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1877      +/-   ##
==========================================
+ Coverage   47.91%   50.94%   +3.02%     
==========================================
  Files         162      192      +30     
  Lines       23491    24961    +1470     
==========================================
+ Hits        11256    12716    +1460     
+ Misses      11014    10941      -73     
- Partials     1221     1304      +83     

Flag	Coverage Δ
unittests	50.94% <100.00%> (+3.02%)	⬆️

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[furykerry]

consider separate logging and priority mutation logic into two commits

[furykerry]

/lgtm
/approve