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 7.79221% with 71 lines in your changes missing coverage. Please review.

Project coverage is 68.73%. Comparing base (ad11af7) to head (df49a51).

Files with missing lines	Patch %	Lines
etcdutl/etcdutl/common.go	0.00%	47 Missing ⚠️
etcdutl/etcdutl/migrate_command.go	0.00%	24 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
server/etcdserver/api/membership/cluster.go	88.57% <100.00%> (+0.04%)	⬆️
etcdutl/etcdutl/migrate_command.go	0.00% <0.00%> (ø)
etcdutl/etcdutl/common.go	16.43% <0.00%> (-29.72%)	⬇️

... and 26 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19168      +/-   ##
==========================================
- Coverage   68.83%   68.73%   -0.10%     
==========================================
  Files         420      420              
  Lines       35641    35703      +62     
==========================================
+ Hits        24535    24542       +7     
- Misses       9680     9745      +65     
+ Partials     1426     1416      -10     

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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?

[k8s-ci-robot]

@ahrtr: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-etcd-robustness-arm64	df49a51	link	true	/test pull-etcd-robustness-arm64

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