feat(flow): add keyed flow implementation

[reugn]

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Codecov Report

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

Project coverage is 99.45%. Comparing base (de0eaa2) to head (30433f3).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #141      +/-   ##
==========================================
+ Coverage   99.40%   99.45%   +0.05%     
==========================================
  Files          11       12       +1     
  Lines         504      552      +48     
==========================================
+ Hits          501      549      +48     
  Misses          3        3              

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