[Do not Merge] [WIP] Combine.perKeyWithBucketing: Combiner for reducing key cardinality

[arunpandianp]

Combine.perKeyWithBucketing(childCombiner, numBuckets) applies the child combiner to the PCollection using numberOfBuckets number of intermediate keys.

This is a POC, sending it now to share and get early feedback.

TODO: Add tests, pick better names.

Example usage

input.apply(Combine.perKeyWithBucketing(yourCombineFn, desiredNumKeys))

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 58.93%. Comparing base (1e27978) to head (768981a). Report is 386 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #32831      +/-   ##
============================================
+ Coverage     57.41%   58.93%   +1.52%     
- Complexity     1475     3102    +1627     
============================================
  Files           968     1131     +163     
  Lines        154224   174643   +20419     
  Branches       1076     3330    +2254     
============================================
+ Hits          88546   102931   +14385     
- Misses        63477    68373    +4896     
- Partials       2201     3339    +1138     

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java	69.91% <ø> (+1.33%)	:arrow_up:

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[arunpandianp]

Closing this in favor of https://github.com/apache/beam/pull/33318