Minor refactor on the etcdutl migrate command

[ahrtr]

Extracted from #19168

I did not change the functionality. Instead It just breaks down the existing (o *migrateOptions) Config() into two methods, moved some code into a separate method (c *migrateConfig) finalize().

It's to make the reviewing on #19168 easier.

cc @fuweid @ivanvc @jmhbnz @serathius

[ahrtr]

/test pull-etcd-unit-test-386

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 20 lines in your changes missing coverage. Please review.

Project coverage is 68.83%. Comparing base (92673d1) to head (39ccd99).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
etcdutl/etcdutl/migrate_command.go	0.00%	20 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
etcdutl/etcdutl/migrate_command.go	0.00% <0.00%> (ø)

... and 21 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19207      +/-   ##
==========================================
+ Coverage   68.79%   68.83%   +0.04%     
==========================================
  Files         420      420              
  Lines       35629    35635       +6     
==========================================
+ Hits        24510    24531      +21     
+ Misses       9700     9685      -15     
  Partials     1419     1419              

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[ahrtr]

/test pull-etcd-robustness-arm64

[k8s-ci-robot]

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[ahrtr]

Thanks @serathius for the quick review!