feat: Accumulator

[yhl25]

fixes #171

[codecov]

Codecov Report

Attention: Patch coverage is 78.16092% with 57 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@cf0c12f). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...maflow/accumulator/AccumulatorSupervisorActor.java	78.12%	13 Missing and 1 partial ⚠️
...java/io/numaproj/numaflow/accumulator/Service.java	68.18%	12 Missing and 2 partials ⚠️
.../java/io/numaproj/numaflow/accumulator/Server.java	60.60%	11 Missing and 2 partials ⚠️
...o/numaproj/numaflow/accumulator/ShutdownActor.java	52.38%	10 Missing ⚠️
.../io/numaproj/numaflow/accumulator/OutputActor.java	80.00%	2 Missing ⚠️
...numaflow/accumulator/OutputStreamObserverImpl.java	94.87%	0 Missing and 2 partials ⚠️
...va/io/numaproj/numaflow/accumulator/Constants.java	0.00%	1 Missing ⚠️
...a/io/numaproj/numaflow/accumulator/GRPCConfig.java	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #172   +/-   ##
=======================================
  Coverage        ?   59.93%           
  Complexity      ?      461           
=======================================
  Files           ?      150           
  Lines           ?     3312           
  Branches        ?      200           
=======================================
  Hits            ?     1985           
  Misses          ?     1194           
  Partials        ?      133           

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[KeranYang]

LGTM!