schemadiff/Online DDL internal refactor

[shlomi-noach]

Description

Followup to the work in #16462, moving (and not changing) logic outside vttablet/onlineddl/executor.go and into schemadiff/onlineddl.go. Functions + tests move across packages, and function arguments change a bit because we don't want schemadiff to rely on schema.OnlineDDL, but otherwise no changes made.

Related Issue(s)

• Online DDL: switch unique key and column logic to declarative schemadiff analysis #16229
• 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]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 86.95652% with 15 lines in your changes missing coverage. Please review.

Project coverage is 68.94%. Comparing base (d9e59b6) to head (05d751c).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/schemadiff/onlineddl.go	89.24%	10 Missing ⚠️
go/vt/vttablet/onlineddl/executor.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16767      +/-   ##
==========================================
- Coverage   68.94%   68.94%   -0.01%     
==========================================
  Files        1565     1565              
  Lines      201764   201765       +1     
==========================================
- Hits       139112   139105       -7     
- Misses      62652    62660       +8     

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