support karmadactl config current-context

[zhzhuang-zju]

What type of PR is this?
/king feature

What this PR does / why we need it:
Support command karmadactl config current-context to view the current-context
Which issue(s) this PR fixes:
Parts of #4297

Special notes for your reviewer:
NONE
Does this PR introduce a user-facing change?:

add command ```karmadactl config current-context``` to view the current-context

[karmada-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4fe9dff) 52.78% compared to head (2607653) 52.78%.
Report is 2 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4299   +/-   ##
=======================================
  Coverage   52.78%   52.78%           
=======================================
  Files         240      240           
  Lines       23624    23624           
=======================================
+ Hits        12470    12471    +1     
+ Misses      10474    10473    -1     
  Partials      680      680           

Flag	Coverage Δ
unittests	52.78% <ø> (+<0.01%)	⬆️

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