Additional recursive CTE work

[systay]

Description

Issues in this first implementation found and fixed in this PR. They include:

• Correctly handle the case when we only use the CTE table in the RHS of the recursive UNION
• Merging CTEs in more situations
• Handle UNION DISTINCT in the CTE definition
• Stop predicate push down

Related Issue(s)

Part of #16415

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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[codecov]

Codecov Report

Attention: Patch coverage is 68.57143% with 11 lines in your changes missing coverage. Please review.

Project coverage is 68.96%. Comparing base (538dd4c) to head (3fc86a4).
Report is 8 commits behind head on main.

Files	Patch %	Lines
go/vt/vtgate/planbuilder/operators/recurse_cte.go	64.28%	5 Missing ⚠️
go/vt/vtgate/planbuilder/operators/SQL_builder.go	55.55%	4 Missing ⚠️
go/vt/vtgate/planbuilder/operators/cte_merging.go	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16616      +/-   ##
==========================================
- Coverage   68.98%   68.96%   -0.02%     
==========================================
  Files        1562     1562              
  Lines      200690   200744      +54     
==========================================
+ Hits       138449   138451       +2     
- Misses      62241    62293      +52     

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[GuptaManan100]

LGTM!

[harshit-gangal]

add a comment when the statement type be nil