support unregister cluster in karmada

[wulemao]

What type of PR is this?

/kind feature

What this PR does / why we need it:

support unregister cluster in karmada

Which issue(s) this PR fixes:

Fixes part of #5380

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

karmadactl: support unregister a pull mode cluster

[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 chaunceyjiang for approval. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• pkg/karmadactl/OWNERS
• pkg/util/OWNERS

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[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 148 lines in your changes missing coverage. Please review.

Project coverage is 35.09%. Comparing base (eaa3452) to head (f223e1b).
Report is 18 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/karmadactl/unregister/unregister.go	0.00%	119 Missing ⚠️
pkg/karmadactl/util/cluster.go	0.00%	22 Missing ⚠️
pkg/util/secret.go	0.00%	5 Missing ⚠️
pkg/karmadactl/karmadactl.go	0.00%	1 Missing ⚠️
pkg/karmadactl/unjoin/unjoin.go	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5626      +/-   ##
==========================================
+ Coverage   35.01%   35.09%   +0.07%     
==========================================
  Files         645      647       +2     
  Lines       44861    44994     +133     
==========================================
+ Hits        15708    15789      +81     
- Misses      27938    27973      +35     
- Partials     1215     1232      +17     

Flag	Coverage Δ
unittests	35.09% <0.00%> (+0.07%)	⬆️

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