Online DDL: refactor schema and migration analysis into declarative schemadiff logic

[shlomi-noach]

Description

This PR refactors the following logic out of onlineddl/executor/vrepl and into schemadiff, or otherwise simplifies it on onlineddl's side of things:

• Reading table columns
• Analyzing column types
• Analyzing column expansion (used for revertiblity)
• Reading and choosing appropriate unique keys for migration
• Reading table's auto_increment value

This is work in progress. Will add more info later on.

Related Issue(s)

Fixes #16229

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 77.07101% with 155 lines in your changes missing coverage. Please review.

Project coverage is 68.76%. Comparing base (3186413) to head (60ed533).
Report is 5 commits behind head on main.

Files	Patch %	Lines
go/vt/vttablet/onlineddl/vrepl.go	27.63%	55 Missing ⚠️
go/vt/schemadiff/onlineddl.go	89.56%	31 Missing ⚠️
go/vt/vttablet/onlineddl/executor.go	0.00%	29 Missing ⚠️
go/vt/schemadiff/column.go	82.60%	24 Missing ⚠️
go/vt/schemadiff/key.go	83.07%	11 Missing ⚠️
go/vt/schemadiff/table.go	90.74%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16256      +/-   ##
==========================================
+ Coverage   68.66%   68.76%   +0.09%     
==========================================
  Files        1548     1546       -2     
  Lines      199124   199271     +147     
==========================================
+ Hits       136731   137024     +293     
+ Misses      62393    62247     -146     

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[shlomi-noach]

Replaced by #16462