DDL allowed outside of transaction in vttablet

[harshit-gangal]

Description

This PR is follow up on #14110
It basically adds a check to disallow DDL execution inside an ongoing transaction.

VTGate is suppose to implicit commit the existing transaction before sending the DDL statement down to vttablet.

VTGate did not handle the case on shard targeting. It is covered now in this PR.

Related Issue(s)

• Closes Bug Report: DDL execution leaves an open transaction #14111

Checklist

• 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

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[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 68.94%. Comparing base (5ced946) to head (4f8f162).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/engine/send.go	75.00%	2 Missing ⚠️
go/vt/vttablet/tabletserver/query_executor.go	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16661      +/-   ##
==========================================
+ Coverage   68.92%   68.94%   +0.01%     
==========================================
  Files        1565     1565              
  Lines      201682   201748      +66     
==========================================
+ Hits       139010   139089      +79     
+ Misses      62672    62659      -13     

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[shlomi-noach]

Makes sense to me.