refactor: change FuncExpr to use Exprs instead of SelectExprs

[systay]

Description

A pure refactoring. Functions (FuncExpr)were accepting the interface SelectExpr, because we used this for count(*). That need is no longer there, since we have separate parsing and AST struct for count(*) now.

This PR just changes FuncExpr to use pure expressions and not aliased expressions.

Related Issue(s)

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]

Review Checklist

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[GuptaManan100]

Love it

[codecov]

Codecov Report

Attention: Patch coverage is 63.15789% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 65.42%. Comparing base (696fe0e) to head (8616a7a).
Report is 58 commits behind head on main.

Files	Patch %	Lines
go/vt/vttablet/tabletmanager/vdiff/table_differ.go	0.00%	4 Missing ⚠️
go/vt/wrangler/materializer.go	0.00%	4 Missing ⚠️
go/vt/wrangler/vdiff.go	0.00%	4 Missing ⚠️
go/vt/vtgate/simplifier/expression_simplifier.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15368      +/-   ##
==========================================
- Coverage   67.41%   65.42%   -1.99%     
==========================================
  Files        1560     1561       +1     
  Lines      192752   193584     +832     
==========================================
- Hits       129952   126662    -3290     
- Misses      62800    66922    +4122     

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[deepthi]

Note for reviewers - refactors should be marked as Internal Cleanup so that we don't need to require an Issue. Enhancements are required to have an issue associated with them.