schemadiff: add EntityDiffByStatement, a testing friendly utility

[shlomi-noach]

Description

This very simple PR adds a new utility function, that wraps a sqlparser.Statement with an appropriate EntityDiff. It is useful for unit tests that run outside the schemadiff package, and specifically for those that import schemadiff as a library.

Related Issue(s)

• schemadiff: tracking issue #10203

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

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.72%. Comparing base (a1acdde) to head (bbbe1a7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15519   +/-   ##
=======================================
  Coverage   65.72%   65.72%           
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  Files        1562     1562           
  Lines      194438   194455   +17     
=======================================
+ Hits       127795   127807   +12     
- Misses      66643    66648    +5     

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