[release-20.0-rc] refactor: cleaner cloning API (#16079)

[harshit-gangal]

Description

Backport of #16079
Needed to backport the Regression fix #16049

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

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[codecov]

Codecov Report

Attention: Patch coverage is 88.40580% with 8 lines in your changes missing coverage. Please review.

Project coverage is 68.50%. Comparing base (965da11) to head (2c3d503).
Report is 3 commits behind head on release-20.0-rc.

Files	Patch %	Lines
go/vt/vtgate/simplifier/simplifier.go	71.42%	4 Missing ⚠️
go/vt/vttablet/onlineddl/executor.go	33.33%	2 Missing ⚠️
go/vt/vtgate/planbuilder/operators/table.go	0.00%	1 Missing ⚠️
go/vt/vtgate/vschema_manager.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           release-20.0-rc   #16136      +/-   ##
===================================================
+ Coverage            68.24%   68.50%   +0.25%     
===================================================
  Files                 1541     1541              
  Lines               197330   197472     +142     
===================================================
+ Hits                134665   135273     +608     
+ Misses               62665    62199     -466     

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Technically not needed for RC/GA of v20

[frouioui]

Although the refactor does not seem risky, let's not backport this to 20.0. It is also not needed for the subquery fix #16049 on release branches.