Feature get_execution_result argument of execute and subscribe

[leszekhanusz]

Add new argument get_execution_result to execute and subscribe methods.

If get_execution_result is True, then the full ExecutionResult (with the extension field) is returned to the user.

[codecov-commenter]

Codecov Report

Merging #257 (c000c51) into master (6cfab1e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #257   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         1650      1657    +7     
=========================================
+ Hits          1650      1657    +7     

Impacted Files	Coverage Δ
gql/client.py	100.00% <100.00%> (ø)
gql/transport/exceptions.py	100.00% <100.00%> (ø)

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