Consistent lookup vindex tests for atomic distributed transactions

[GuptaManan100]

Description

This PR adds testing for using consistent lookup vindexes for atomic distributed transactions. It also makes the code changes to make them work, which were initially disallowed. The changes are quite self-explanatory. We do the pre-session and post-session changes like we were doing for the normal commit flow, and hence have been moved out into common code path.

Related Issue(s)

• RFC: Atomic Distributed Transactions #16245

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 90.90909% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.53%. Comparing base (45192d2) to head (6055df4).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/tx_conn.go	90.90%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17393   +/-   ##
=======================================
  Coverage   67.52%   67.53%           
=======================================
  Files        1581     1581           
  Lines      253948   253945    -3     
=======================================
+ Hits       171480   171491   +11     
+ Misses      82468    82454   -14     

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