Online DDL internal cleanup: using formal statements as opposed to textual SQL

[shlomi-noach]

Description

Some internal refactoring, using more format sqlparser.Statement or sqlparser.CreateTable etc. rather than raw SQL text.

Related Issue(s)

Initial work for #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

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

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• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
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• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 8.69565% with 42 lines in your changes missing coverage. Please review.

Project coverage is 68.61%. Comparing base (1ffabca) to head (0d8fc60).
Report is 4 commits behind head on main.

Files	Patch %	Lines
go/vt/vttablet/onlineddl/executor.go	4.34%	22 Missing ⚠️
go/vt/vttablet/onlineddl/vrepl.go	0.00%	19 Missing ⚠️
go/vt/vttablet/onlineddl/vrepl/parser.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16230      +/-   ##
==========================================
+ Coverage   68.55%   68.61%   +0.06%     
==========================================
  Files        1544     1544              
  Lines      197895   197948      +53     
==========================================
+ Hits       135658   135829     +171     
+ Misses      62237    62119     -118     

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