[multistage][feature] revisit RelDistribution detection logic

This discussion stems from: #11976

Status quo
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we currently
1. explicitly insert *logical* exchanges where we *might* require data shuffling; and
2. then determine whether those exchanges are real data shuffle or possible passthrough;
3. then we determine whether to assign more/less servers to run either side of the exchanges

Current solution
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This PR (#11976) only addresses step-2: to give it a better idea on whether the RelDistribution is the same before & after an exchange. for this purpose, it is OK (at this point) to only keep one of the 2 sides for JOIN Rel.

Needs revisit
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there are several problems
1. should we explicitly insert exchange or should we use other abstractions?
- there are other ways to add exchange nodes that are more "Calcite-suggested" when managing the Exchange insert instead of applying them during optimization IMO.
2. should the exchange insert be RelDistribution-based or physical-based?
- we are mixing the concept of Exchange usage: it can mean (1) altering logical RelDistribution, (2) indicating there potentially could be physical layout differences, (3) whether we can apply leaf-stage optimization
- although (3) will be addressed by #11937, we should consider whether to still use ExchangeNode as our abstraction or create our own to avoid confusion
3. should we apply RelDistribution trait before or after Exchange insert
- currently we have to do this after insert, but technically if we addresses question (2) we can potentially apply that beforehand

ultimately utilizing Exchange was a quick decision during early stage multi-stage engine development and it might not have been the best option. it is worth taking some time to revisit

Review the discussion in #11976 and the related work in #11937 first, then trace the multi-stage engine's exchange insertion and RelDistribution handling described here. The work is done when the exchange abstraction, logical versus physical responsibilities, and timing of RelDistribution application have an agreed implementation and validation.

Written by the indexing model from the issue text.