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ARROW-9464: [Rust] [DataFusion] Physical plan optimization rule to insert MergeExec when needed

This PR adds the first physical optimization rule, to insert explicit MergeExec nodes into the physical plan when operators require a single partition of input (such as GlobalLimitExec, SortExec, HashAggregateExec in final mode).

This removes the merging logic from the operators, making them more easily re-usable in different contexts (such as in a distributed query engine, which could provide its own planner and/or optimization rules).

Closes #8034 from andygrove/physical-optimizer-merge

Lead-authored-by: Andy Grove <andygrove73@gmail.com>
Co-authored-by: Andy Grove <andygrove@users.noreply.github.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>