Add error transformer to vtgate executor

[rafer]

Description

This makes easier it for other components to plug in logic to modify errors before they are returned to the client by vtgate. By default the nullTransformer is used, which does nothing.

Related Issue(s)

N/A

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

None

[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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Backward compatibility

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• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.45%. Comparing base (cbf89bd) to head (2d609dd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15894      +/-   ##
==========================================
+ Coverage   68.43%   68.45%   +0.01%     
==========================================
  Files        1559     1559              
  Lines      196753   196760       +7     
==========================================
+ Hits       134657   134689      +32     
+ Misses      62096    62071      -25     

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