… they are not equal

  ### What changes were proposed in this pull request?
-- Allow SPJ between 'compatible' bucket funtions
-- Add a mechanism to define 'reducible' functions, one function whose output can be 'reduced' to another for all inputs.

  ### Why are the changes needed?
-- SPJ currently applies only if the partition transform expressions on both sides are identifical.

  ### Does this PR introduce _any_ user-facing change?
No

  ### How was this patch tested?
Added new tests in KeyGroupedPartitioningSuite

  ### Was this patch authored or co-authored using generative AI tooling?
No