Custom Scalars input serialization in variables

[leszekhanusz]

New feature which implements Custom Scalars serialization in variable_values.

For this feature to work:

• a schema is needed
• the scalars implementation should be added as a GraphQLScalarType in the schema
• the execute or subscribe methods (sync or async) should be called with serialize_variables=True

[codecov-commenter]

Codecov Report

Merging #253 (1047885) into master (d5e3e6d) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #253   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        19    +3     
  Lines         1335      1404   +69     
=========================================
+ Hits          1335      1404   +69     

Impacted Files	Coverage Δ
gql/client.py	100.00% <100.00%> (ø)
gql/utilities/__init__.py	100.00% <100.00%> (ø)
gql/utilities/update_schema_scalars.py	100.00% <100.00%> (ø)
gql/variable_values.py	100.00% <100.00%> (ø)

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