Planner cleaning: cleanup and refactor

[systay]

Description

While working on #16427, I've stumbled across quite a few annoyances in the code that needed fixing before we could finish the CTE work. Instead of making that PR even bigger, I'm extracting all the clean ups into this PR.

The kind of clean ups are:

• Use AddWSColumn instead of AddColumn to add weight_strings.
• Don't push down literals - just evaluate them in the vtgate

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

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

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Documentation

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• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

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• Protobuf changes should be wire-compatible.
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• RPC changes should be compatible with vitess-operator
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• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 97.65625% with 3 lines in your changes missing coverage. Please review.

Project coverage is 68.83%. Comparing base (cc68dd5) to head (4451af6).
Report is 2 commits behind head on main.

Files	Patch %	Lines
go/vt/vtgate/planbuilder/operators/aggregator.go	94.44%	1 Missing ⚠️
go/vt/vtgate/planbuilder/operators/apply_join.go	83.33%	1 Missing ⚠️
...vtgate/planbuilder/operators/projection_pushing.go	96.55%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16569      +/-   ##
==========================================
- Coverage   68.85%   68.83%   -0.03%     
==========================================
  Files        1557     1557              
  Lines      199891   199983      +92     
==========================================
+ Hits       137644   137659      +15     
- Misses      62247    62324      +77     

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[GuptaManan100]

Rest LGTM