Online DDL: --singleton-table DDL strategy flag

[shlomi-noach]

Description

Per #17162, we add a --singleton-table DDL strategy flag, which prevents an Online DDL submission for a table (or view) that is already pending an Online DDL operation.

Related Issue(s)

Fixes #17162

Documentation: vitessio/website#1881

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

Attention: Patch coverage is 25.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 67.44%. Comparing base (e87457e) to head (fc64ff9).

Files with missing lines	Patch %	Lines
go/vt/vttablet/onlineddl/executor.go	0.00%	9 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17169   +/-   ##
=======================================
  Coverage   67.44%   67.44%           
=======================================
  Files        1569     1569           
  Lines      252178   252189   +11     
=======================================
+ Hits       170080   170088    +8     
- Misses      82098    82101    +3     

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