schemadiff: atomic diffs for range partition DROP PARTITION statement

[shlomi-noach]

Description

Introducing a AtomicDiffs() function, which, given a single diff, returns a slice of diffs, as atomic as possible, which together comprises the given diff.

Right now the function only breaks down DROP PARTITION statements.

An an aside, this PR also adds a convenient Clone() function to EntityDiff interface, which does a deep copy of the diff and of its entities (but not necessarily other constructs such as annotations). This is useful for programs using schemadiff as a library as it allows some safety for data manipulation.

Related Issue(s)

Addresses #15842

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[dbussink]

Let's start here and we can further look into what else we want to split up for AtomicDiffs. I suspect there's probably some more cases where it's useful but we can do that in a separate change.

[codecov]

Codecov Report

Attention: Patch coverage is 91.35802% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 68.46%. Comparing base (f118ba2) to head (4cb50fc).
Report is 71 commits behind head on main.

Files	Patch %	Lines
go/vt/schemadiff/table.go	90.24%	4 Missing ⚠️
go/vt/schemadiff/view.go	86.95%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15843      +/-   ##
==========================================
+ Coverage   68.40%   68.46%   +0.05%     
==========================================
  Files        1556     1559       +3     
  Lines      195121   196592    +1471     
==========================================
+ Hits       133479   134589    +1110     
- Misses      61642    62003     +361     

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