Restrict the cluster sync mode for the unjoin and unregister command

[zhzhuang-zju]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Restrict the cluster sync mode for the unjoin and unregister command.

$ karmadactl get cluster
NAME      CLUSTER   VERSION   MODE   READY   AGE     ADOPTION
member1   Karmada   v1.31.2   Push   True    17h     -
member3   Karmada   v1.31.2   Pull   True    3d23h   -
$ karmadactl unregister member1  --cluster-kubeconfig ~/.kube/members.config --karmada-config ~/.kube/karmada.config --cluster-context member1 -v=4 
I0124 11:10:55.943833 2452252 unregister.go:188] Unregistering cluster. cluster name: member1
I0124 11:10:55.943887 2452252 unregister.go:189] Unregistering cluster. karmada-agent deployed in namespace: karmada-system
I0124 11:10:55.943901 2452252 unregister.go:190] Unregistering cluster. member cluster secrets stored in namespace: karmada-cluster
error: cluster member1 is a Push mode member cluster, please use command `unjoin` if you want to unregister cluster
$ karmadactl unjoin member3 --cluster-kubeconfig=/root/.kube/members.config --cluster-context=member3                                               
error: cluster member3 is a Pull mode member cluster, please use command `unregister` if you want to unregister cluster

Which issue(s) this PR fixes:
Fixes #801

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

`karmadactl`: The `unjoin` command is restricted to only unjoin push mode member clusters. The `unregister` command is restricted to only unregister pull mode member clusters.

[karmada-bot]

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[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 38.46154% with 16 lines in your changes missing coverage. Please review.

Project coverage is 48.31%. Comparing base (3e3e8fe) to head (99d985a).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/karmadactl/util/cluster.go	0.00%	10 Missing ⚠️
pkg/karmadactl/unjoin/unjoin.go	62.50%	2 Missing and 1 partial ⚠️
pkg/karmadactl/unregister/unregister.go	62.50%	2 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6081      +/-   ##
==========================================
- Coverage   48.35%   48.31%   -0.05%     
==========================================
  Files         666      666              
  Lines       54874    54903      +29     
==========================================
- Hits        26535    26527       -8     
- Misses      26615    26645      +30     
- Partials     1724     1731       +7     

Flag	Coverage Δ
unittests	48.31% <38.46%> (-0.05%)	⬇️

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[RainbowMango]

error: cluster member1 is a Push mode member cluster, please use command unjoin if you want to unregister cluster
error: cluster member3 is a Pull mode member cluster, please use command unregister if you want to unregister cluster

No data was changed or removed before this log, right?

[zhzhuang-zju]

No data was changed or removed before this log, right?

Yes, I put the validations ahead of the unjoin/unregister operations