Create a v2 snapshot when running etcdutl migrate command

[ahrtr]

Refer to #17911 (comment)

This PR will make the etcdutl migrate command fully functional.

• It creates a v2snapshot from the v3store.
  You will never see error below anymore when executing etcdutl migrate command,

  Error: cannot downgrade storage, WAL contains newer entries, as the target version (3.5.0) is lower than the version (3.6.0) detected from WAL logs
  

  After executing the migrate command for all members, you just need to directly replace the binary of each member, then all done for the offline downgrade. Of course, it's still recommended to follow/perform the online downgrade process, as it doesn't break the workload. cc @ivanvc @jmhbnz

• It also adds a separate etcdutl v2snapshot create command
  It's just a manual last to resort solution for any potential issue. Usually we don't need it.

I need to add e2e test. I may also break down it into smaller PRs.

[k8s-ci-robot]

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[codecov]

Codecov Report

Attention: Patch coverage is 67.79661% with 19 lines in your changes missing coverage. Please review.

Project coverage is 68.76%. Comparing base (ab819b5) to head (e3fb899).

Files with missing lines	Patch %	Lines
etcdutl/etcdutl/common.go	76.92%	6 Missing and 6 partials ⚠️
etcdutl/etcdutl/migrate_command.go	0.00%	7 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
etcdutl/etcdutl/migrate_command.go	0.00% <0.00%> (ø)
etcdutl/etcdutl/common.go	74.35% <76.92%> (+28.20%)	⬆️

... and 24 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19168      +/-   ##
==========================================
- Coverage   68.84%   68.76%   -0.08%     
==========================================
  Files         420      420              
  Lines       35635    35693      +58     
==========================================
+ Hits        24533    24545      +12     
- Misses       9686     9723      +37     
- Partials     1416     1425       +9     

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[ahrtr]

This is a huge PR, let me breakdown it into small PRs to make the review easier.

[serathius]

So many codecov warnings, is there any way to hide them?

[ahrtr]

So many codecov warnings, is there any way to hide them?

Added an unit test, PTAL.

cc @fuweid @serathius @siyuanfoundation

[ahrtr]

/retest

[ahrtr]

/test pull-etcd-robustness-arm64