Feature execute batch async

[leszekhanusz]

Implementation of batching (See issue #430) for async transports (aiohttp and httpx), and for the sync httpx transport too.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7fb869a) to head (d26abd5).
Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master      #550    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           38        40     +2     
  Lines         2908      3028   +120     
==========================================
+ Hits          2908      3028   +120     

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