feat: Add label on the namespace created by karmada

[zhy76]

What type of PR is this?

What this PR does / why we need it:

Which issue(s) this PR fixes:
Fixes #2862

Special notes for your reviewer:
when adding a cluster to karmada, it will has a label indicating that the namespace was created by karmada to avoid accidental deletion.:

Does this PR introduce a user-facing change?:

karmadactl join/register, karmada-agent: Add labels on the namespace created by karmada.

[codecov-commenter]

Codecov Report

Merging #3839 (8c25212) into master (2526de8) will increase coverage by 0.04%.
Report is 2 commits behind head on master.
The diff coverage is 87.50%.

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@@            Coverage Diff             @@
##           master    #3839      +/-   ##
==========================================
+ Coverage   55.17%   55.22%   +0.04%     
==========================================
  Files         227      227              
  Lines       21653    21666      +13     
==========================================
+ Hits        11948    11965      +17     
+ Misses       9069     9065       -4     
  Partials      636      636              

Flag	Coverage Δ
unittests	55.22% <87.50%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
pkg/util/credential.go	65.28% <80.00%> (+2.45%)	⬆️
pkg/util/namespace.go	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

[XiShanYongYe-Chang]

Thanks a lot
/lgtm

[XiShanYongYe-Chang]

/kind feature

Can you help update the release note:

karmadactl join/register, karmada-agent: Add labels on the namespace created by karmada.

[zhy76]

/kind feature

Can you help update the release note:

karmadactl join/register, karmada-agent: Add labels on the namespace created by karmada.

OK

[whitewindmills]

/lgtm

[RainbowMango]

/assign

[XiShanYongYe-Chang]

Thanks~

[XiShanYongYe-Chang]

So quick response.
/lgtm

[RainbowMango]

/approve

Thanks.

[karmada-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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