Implementation of automatic batching for async

[leszekhanusz]

See #430

Now that execute_batch is supported on async transports, we implement the automatic batching of requests which is activated by setting the batching_interval parameter of the Client.

If multiple requests are executed in different asyncio tasks in the interval, they will be automatically batched in a single request.

• Adding docs
• now supporting using a str instead of a document to simplify making the GraphQLRequest object.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7fb869a) to head (d02bed3).
Report is 17 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master      #554    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           38        40     +2     
  Lines         2908      3084   +176     
==========================================
+ Hits          2908      3084   +176     

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