Online DDL: support alter vitess_migration ... postpone complete syntax

[shlomi-noach]

Description

This PR introduces two new queries:

• alter vitess_migration '<uuid>' postpone complete
• alter vitess_migration postpone complete all

With these two queries, it is possible to set postpone_completion on already running migrations that are not postponed. The use case in particular is for when a migration seems to run into an endless cut-over sequence and is unable to complete. It may be desired to withhold completion until further notice.

These commands are the opposite of alter vitess_migration ... complete [all] commands, and can alternate back and forth multiple times in the lifetime of the migration.

Related Issue(s)

• Closes Feature Request: Add support for stopping OnlineDDL complete #18107
• docs: Docs: alter vitess_migration ... postpone complete website#1991

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 0% with 54 lines in your changes missing coverage. Please review.

Project coverage is 67.45%. Comparing base (9e734f8) to head (dde5696).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/onlineddl/executor.go	0.00%	46 Missing ⚠️
go/vt/sqlparser/ast_format.go	0.00%	4 Missing ⚠️
go/vt/vttablet/tabletserver/query_executor.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18118      +/-   ##
==========================================
- Coverage   67.53%   67.45%   -0.09%     
==========================================
  Files        1601     1601              
  Lines      261484   262150     +666     
==========================================
+ Hits       176592   176828     +236     
- Misses      84892    85322     +430     

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[shlomi-noach]

Changed syntax from alter vitess_migration ... postpone to alter vitess_migration ... postpone complete for consistency with --postpone-complete ddl strategy flag.

[shlomi-noach]

Docs PR: vitessio/website#1991