Fix panic in aggregation

[GuptaManan100]

Description

When using the aggregation engine, we need to have full field information from the inputs to calculate things correctly.

This PR makes sure that the aggregator always asks for fields. When it was running inside of some other operators that don't need fields, we failed to ask for the fields, and this could lead to panics.

Related Issue(s)

• Fixes Bug Report: Panic in tpch query run #15729

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]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.39%. Comparing base (f118ba2) to head (79605bc).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15728      +/-   ##
==========================================
- Coverage   68.40%   68.39%   -0.01%     
==========================================
  Files        1556     1556              
  Lines      195121   195347     +226     
==========================================
+ Hits       133479   133617     +138     
- Misses      61642    61730      +88     

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[GrahamCampbell]

Can this be backported to v19?