refactor: VTGate executor with a runnable context package

[harshit-gangal]

Description

This PR refactor the VTGate executor to have session and vcursor in executorcontext package so that planbuilder can use the common VCursor implementation in the plan tests.

Related Issue(s)

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

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 81.02073% with 119 lines in your changes missing coverage. Please review.

Project coverage is 67.41%. Comparing base (2da3893) to head (696d5e2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/executorcontext/vcursor_impl.go	75.82%	88 Missing ⚠️
go/vt/vtgate/executorcontext/safe_session.go	70.17%	17 Missing ⚠️
go/vt/vtgate/scatter_conn.go	81.48%	5 Missing ⚠️
go/vt/vtgate/executor.go	95.55%	4 Missing ⚠️
go/test/vschemawrapper/vschema_wrapper.go	89.47%	2 Missing ⚠️
go/vt/vtgate/executorcontext/faketopo.go	91.30%	2 Missing ⚠️
go/vt/vtgate/vtgate.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17305      +/-   ##
==========================================
- Coverage   67.42%   67.41%   -0.02%     
==========================================
  Files        1575     1576       +1     
  Lines      253324   253417      +93     
==========================================
+ Hits       170816   170837      +21     
- Misses      82508    82580      +72     

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[vitess-bot]

Hello! 👋

This Pull Request is now handled by arewefastyet. The current HEAD and future commits will be benchmarked.

You can find the performance comparison on the arewefastyet website.

[systay]

very nice! being able to move to an interface here is an important step

[GuptaManan100]

Just a nomenclature thing. Totally your call.