Add extensions field to ExecutionResult (#188)

[extrin]

Add the extensions field of the ExecutionResult object for all transports.

[codecov-io]

Codecov Report

Merging #190 (1e3fff8) into master (459d5eb) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #190   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines          949       949           
=========================================
  Hits           949       949           

Impacted Files	Coverage Δ
gql/transport/phoenix_channel_websockets.py	100.00% <ø> (ø)
gql/transport/websockets.py	100.00% <ø> (ø)
gql/transport/aiohttp.py	100.00% <100.00%> (ø)
gql/transport/requests.py	100.00% <100.00%> (ø)

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[leszekhanusz]

Sorry for the delay.
Could you please add some tests for this functionality?
Thanks

[codecov-commenter]

Codecov Report

Merging #190 (ee313a6) into master (459d5eb) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #190   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        16    +2     
  Lines          949       978   +29     
=========================================
+ Hits           949       978   +29     

Impacted Files	Coverage Δ
gql/transport/phoenix_channel_websockets.py	100.00% <ø> (ø)
gql/transport/websockets.py	100.00% <ø> (ø)
gql/transport/aiohttp.py	100.00% <100.00%> (ø)
gql/transport/requests.py	100.00% <100.00%> (ø)
gql/utils.py	100.00% <0.00%> (ø)
gql/__init__.py	100.00% <0.00%> (ø)

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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