[Solution 2] Add DowngradeVersionTestRequest for Downgrade or migration test only

[ahrtr]

Link to #19274

Please review this PR commit by commit, it should be super clear and straightforward.

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 68.93%. Comparing base (f7a36a5) to head (7b4b96d).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
server/storage/wal/version.go	42.85%	2 Missing and 2 partials ⚠️
server/etcdserver/server.go	33.33%	2 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
server/etcdserver/server.go	82.00% <33.33%> (-0.28%)	⬇️
server/storage/wal/version.go	51.53% <42.85%> (-0.39%)	⬇️

... and 23 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19276      +/-   ##
==========================================
+ Coverage   68.81%   68.93%   +0.12%     
==========================================
  Files         420      420              
  Lines       35680    35689       +9     
==========================================
+ Hits        24552    24602      +50     
+ Misses       9702     9672      -30     
+ Partials     1426     1415      -11     

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[ahrtr]

cc @fuweid @serathius @siyuanfoundation This PR might be a little better than #19274

[ahrtr]

The solution 1 #19274 changes the protoc between client and server, so I prefer to this PR (solution 2). See my summary in #19274 (comment)

cc @fuweid @siyuanfoundation @serathius

[ahrtr]

@fuweid @siyuanfoundation @serathius this is one of the blockers to cut release-3.6, PTAL

[fuweid]

LGTM

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[ahrtr]

Thanks both @fuweid and @siyuanfoundation for the quick review!