Move getLatestWALSnap into etcdutl/common.go

[ahrtr]

This PR is extracted from #19168

cc @fuweid @serathius @ivanvc @jmhbnz @siyuanfoundation

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 6 lines in your changes missing coverage. Please review.

Project coverage is 68.77%. Comparing base (9db8dcb) to head (8374966).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
etcdutl/etcdutl/common.go	66.66%	3 Missing and 3 partials ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
etcdutl/etcdutl/migrate_command.go	0.00% <ø> (-9.79%)	⬇️
etcdutl/etcdutl/common.go	46.15% <66.66%> (+46.15%)	⬆️

... and 24 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19173      +/-   ##
==========================================
- Coverage   68.87%   68.77%   -0.10%     
==========================================
  Files         420      420              
  Lines       35649    35654       +5     
==========================================
- Hits        24554    24522      -32     
- Misses       9674     9706      +32     
- Partials     1421     1426       +5     

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