Gen4 Planner: support aggregate UDFs

[systay]

Description

The schema tracker can now tell us which aggregate UDFs exist on the underlying MySQL, and with this PR the planner will now use this information when planning queries.

It changes how the column binding is done in the HAVING and ORDER BY clause in a subtle but important way.

Vtgate can only handle these functions when it's possible to entirely push down the aggregation to mysql.

At the moment there is no support for actually installing the UDFs on MySQL - for now this exercise is left to the user.

Related Issue(s)

#15705

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• 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 93.28358% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 68.40%. Comparing base (f118ba2) to head (8162d43).
Report is 9 commits behind head on main.

Files	Patch %	Lines
...vt/vtgate/planbuilder/operators/queryprojection.go	86.66%	4 Missing ⚠️
go/vt/vtgate/vindexes/vschema.go	86.66%	2 Missing ⚠️
go/vt/vtgate/planbuilder/operators/aggregator.go	83.33%	1 Missing ⚠️
go/vt/vtgate/planbuilder/operators/hash_join.go	50.00%	1 Missing ⚠️
go/vt/wrangler/vexec_plan.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15710      +/-   ##
==========================================
- Coverage   68.40%   68.40%   -0.01%     
==========================================
  Files        1556     1556              
  Lines      195121   195393     +272     
==========================================
+ Hits       133479   133650     +171     
- Misses      61642    61743     +101     

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[harshit-gangal]

Looks good overall

[GuptaManan100]

LGTM