[spool] Define equivalence between stages

[gortiz]

This PR includes the code that defines when two stages are equivalent and it is the first step to implement #14196.

Instead of implementing the ability to reuse common expressions in a single and very large PR, I decided to create this first one where:

• The new code is not being called by production code (yet)
• Classes are well tested and documented

My hope is that this will be easier to review.

[gortiz]

cc @bziobrowski @vrajat

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 44.60784% with 113 lines in your changes missing coverage. Please review.

Project coverage is 63.75%. Comparing base (59551e4) to head (7758934). Report is 1307 commits behind head on master.

Files with missing lines	Patch %	Lines
.../query/planner/logical/EquivalentStagesFinder.java	40.17%	52 Missing and 18 partials :warning:
.../pinot/query/planner/plannode/PlanNodeVisitor.java	40.00%	20 Missing and 1 partial :warning:
...che/pinot/query/planner/logical/GroupedStages.java	75.00%	7 Missing and 3 partials :warning:
...anner/logical/ParentToChildrenStageCalculator.java	0.00%	8 Missing :warning:
.../apache/pinot/query/planner/plannode/PlanNode.java	0.00%	3 Missing :warning:
.../pinot/query/planner/plannode/MailboxSendNode.java	0.00%	1 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #14296      +/-   ##
============================================
+ Coverage     61.75%   63.75%   +2.00%     
- Complexity      207     1569    +1362     
============================================
  Files          2436     2665     +229     
  Lines        133233   146223   +12990     
  Branches      20636    22432    +1796     
============================================
+ Hits          82274    93221   +10947     
- Misses        44911    46087    +1176     
- Partials       6048     6915     +867     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (+99.99%)	:arrow_up:
integration	100.00% <ø> (+99.99%)	:arrow_up:
integration1	100.00% <ø> (+99.99%)	:arrow_up:
integration2	0.00% <ø> (ø)
java-11	63.71% <44.60%> (+2.00%)	:arrow_up:
java-21	63.64% <44.60%> (+2.01%)	:arrow_up:
skip-bytebuffers-false	63.74% <44.60%> (+1.99%)	:arrow_up:
skip-bytebuffers-true	63.59% <44.60%> (+35.87%)	:arrow_up:
temurin	63.75% <44.60%> (+2.00%)	:arrow_up:
unittests	63.74% <44.60%> (+2.00%)	:arrow_up:
unittests1	55.51% <44.60%> (+8.62%)	:arrow_up:
unittests2	34.09% <0.00%> (+6.36%)	:arrow_up:

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[yashmayya]

I think this branch needs to be rebased / merged with master, all the builds are currently failing (at least one reason seems to be conflict with https://github.com/apache/pinot/pull/14294/).