Allow for dynamic configuration of DDL settings

[dbussink]

This allows configuring the vtgate DDL flags dynamically. With this, both online DDL and direct DDL can be disabled and enabled in vtgate without having to restart.

Related Issue(s)

Fixes #17327

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

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 10 lines in your changes missing coverage. Please review.

Project coverage is 67.41%. Comparing base (f0c0a70) to head (f10a1e4).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/vtgate.go	0.00%	4 Missing ⚠️
go/vt/vtgate/planbuilder/builder.go	76.92%	3 Missing ⚠️
go/vt/vtgate/planbuilder/migration.go	66.66%	2 Missing ⚠️
go/vt/vtgate/engine/ddl.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17328      +/-   ##
==========================================
+ Coverage   67.39%   67.41%   +0.02%     
==========================================
  Files        1574     1575       +1     
  Lines      253299   253310      +11     
==========================================
+ Hits       170708   170768      +60     
+ Misses      82591    82542      -49     

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[GuptaManan100]

LGTM!