Update endpointsFromCluster to reuse clientConfigFromCmd

[ahrtr]

Minor refactoring... try to reuse the common function so as to remove duplication.

Followup of #18723

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.91%. Comparing base (54db7f0) to head (53acdd4).

❗ Current head 53acdd4 differs from pull request most recent head b757772

Please upload reports for the commit b757772 to get more accurate results.

Files with missing lines	Patch %	Lines
etcdctl/ctlv3/command/ep_command.go	0.00%	2 Missing ⚠️

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Additional details and impacted files

Files with missing lines	Coverage Δ
etcdctl/ctlv3/command/ep_command.go	0.00% <0.00%> (ø)

... and 21 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #18742      +/-   ##
==========================================
+ Coverage   68.79%   68.91%   +0.11%     
==========================================
  Files         420      420              
  Lines       35493    35481      -12     
==========================================
+ Hits        24419    24450      +31     
+ Misses       9648     9612      -36     
+ Partials     1426     1419       -7     

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[ahrtr]

/test pull-etcd-integration-1-cpu-amd64

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, serathius

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