Adds events/batch/singular

[JoshVanL]

Adds Singular Batcher which treat all events as the same event key. Subscribers provide a function when subscribing which will be executed once the batch triggers.

Used by https://github.com/dapr/components-contrib/pull/3363

[codecov-commenter]

Codecov Report

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

Project coverage is 79.95%. Comparing base (9e733a3) to head (fa869e2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
+ Coverage   79.76%   79.95%   +0.18%     
==========================================
  Files          55       56       +1     
  Lines        4339     4365      +26     
==========================================
+ Hits         3461     3490      +29     
+ Misses        732      730       -2     
+ Partials      146      145       -1     

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