Introduce a validation webhook to prohibit WorkloadRebalancer from being modified

[chaosi-zju]

DO NOT MERGE until the following PR have been merged (this PR needs a rebasing) :

• Introduce a new API named WorkloadRebalancer to support rescheduling #4841

What type of PR is this?

/kind feature

What this PR does / why we need it:

Introduce a validation webhook to prohibit WorkloadRebalancer from being modified, the final effect is like:

$ kubectl --context karmada-apiserver edit wr demo
error: workloadrebalancers.apps.karmada.io "demo" could not be patched: admission webhook "workloadrebalancer.karmada.io" denied the request: the spec field should not be modified
You can run `kubectl replace -f /tmp/kubectl-edit-vdiqj.yaml` to try this update again.
$
$ kubectl --context karmada-apiserver replace -f demo2.yaml     
Error from server (Forbidden): error when replacing "demo2.yaml": admission webhook "workloadrebalancer.karmada.io" denied the request: the spec field should not be modified

Which issue(s) this PR fixes:

Fixes part of #4840

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[karmada-bot]

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.97%. Comparing base (aded7c0) to head (09f93bd).
Report is 2 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4859      +/-   ##
==========================================
- Coverage   52.99%   52.97%   -0.02%     
==========================================
  Files         250      250              
  Lines       20420    20421       +1     
==========================================
- Hits        10821    10819       -2     
- Misses       8880     8882       +2     
- Partials      719      720       +1     

Flag	Coverage Δ
unittests	52.97% <100.00%> (-0.02%)	⬇️

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[chaosi-zju]

/close

we will try to implemente update event in #4860

[karmada-bot]

@chaosi-zju: Closed this PR.

In response to this:

/close

we will try to implemente update event in #4860

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