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.

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[codecov]

Codecov Report

Attention: Patch coverage is 15.65217% with 97 lines in your changes missing coverage. Please review.

Project coverage is 68.77%. Comparing base (9db8dcb) to head (033f4cf).

Files with missing lines	Patch %	Lines
etcdutl/etcdutl/common.go	18.46%	50 Missing and 3 partials ⚠️
etcdutl/etcdutl/migrate_command.go	0.00%	22 Missing ⚠️
etcdutl/etcdutl/v2snapshot_command.go	0.00%	21 Missing ⚠️
etcdutl/ctl.go	0.00%	1 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/ctl.go	0.00% <0.00%> (ø)
etcdutl/etcdutl/v2snapshot_command.go	0.00% <0.00%> (ø)
etcdutl/etcdutl/migrate_command.go	0.00% <0.00%> (-9.79%)	⬇️
etcdutl/etcdutl/common.go	16.43% <18.46%> (+16.43%)	⬆️

... and 20 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19168      +/-   ##
==========================================
- Coverage   68.87%   68.77%   -0.11%     
==========================================
  Files         420      421       +1     
  Lines       35649    35736      +87     
==========================================
+ Hits        24554    24576      +22     
- Misses       9674     9741      +67     
+ Partials     1421     1419       -2     

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