Store a list of input WhereFilterSpecs in the WhereConstraintNode

[tlento]

Up until now we've been merging sets of WhereFilterSpec instances
into a single instance and then storing that in the WhereConstraintNode.

This made sense for rendering, but when trying to do predicate pushdown
having the specs all merged together limits the space of filter pushdown
opporunities. In order to allow for the same breadth of predicate pushdown
opportunities we have at dataflow plan build time we keep the specs separate,
and encapsulate the merging of these specs into the WhereConstraintNode itself.

[github-actions]

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[tlento]

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• Move categorical dimension filter pushdown to PredicatePushdownOptimizer #1286
• Move node dataset resolver subquery ids into a separate namespace #1280
• Verify column sets match in optimizer pushdown evaluation #1279
• Update API for requesting dataflow plan optimization #1278
• Store a list of input WhereFilterSpecs in the WhereConstraintNode #1277 👈
• Enable DataflowPlanOptimizers for query rendering tests #1263
• Add PredicatePushdownOptimizer in tracking-only mode #1262
• Add integration tests for filters against various join types #1240
• Enable predicate pushdown for categorical dimension filters #1227
• Add semantic_model_origin property to LinkableElement interface #1230
• Add element type property to LinkableElement interface #1226
• Encapsulate time range constraint application in more general method #1225
• Rename PredicatePushdownParameters to PredicatePushdownState #1224
• Make source_semantic_models property accessible from a DataflowPlanNode #1218
• Use pushdown params to disable time range constraint pushdown #1216 : 1 other dependent PR (#1217 )
• Encapsulate time range constraints for predicate pushdown #1215
• Add conversion metric rendering tests #1214
• main

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[courtneyholcomb]

LGTM!