Add key param support for helper Combiners

[harrisonlimh]

addresses #25888

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[codecov]

Codecov Report

Merging #25895 (d2254a1) into master (ddae966) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head d2254a1 differs from pull request most recent head 643ea83. Consider uploading reports for the commit 643ea83 to get more accurate results

@@            Coverage Diff             @@
##           master   #25895      +/-   ##
==========================================
- Coverage   71.42%   71.41%   -0.01%     
==========================================
  Files         779      779              
  Lines      102534   102534              
==========================================
- Hits        73237    73227      -10     
- Misses      27837    27847      +10     
  Partials     1460     1460              

Flag	Coverage Δ
python	79.96% <100.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
sdks/python/apache_beam/transforms/combiners.py	93.43% <100.00%> (ø)

... and 7 files with indirect coverage changes

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[tvalentyn]

given we force KW-only arguments, we can move key before reverse, same as we do in other combiners.

[tvalentyn]

please fix lint, formatter and add a note to CHANGES.md

[tvalentyn]

noting that tests passed.

[tvalentyn]

Thanks, @harrisonlimh !