Feature request: support for custom json serialization

[markedwards]

It would be useful to be able to specify a custom json_dumps callable for the transport. Right now gql is hardcoded to use json.dumps from the standard library, and that has performance issues as well as poor serialization. It is common to have to provide Enum or datetime objects in variables, for instance, and without the ability to specify a custom callable its necessary to pre-process values which is inefficient and annoying to implement.

Any appetite for such a capability in the future?

[leszekhanusz]

What is going to be implemented next is custom scalars, this will work only if the schema is available locally.

You'll define a GraphQLScalarType to define the serialize/unserialize methods and add these custom scalars to the schema, then during the execute, the variables will be e̶n̶c̶o̶d̶e̶d̶ ̶ a̶s̶t̶_̶f̶r̶o̶m̶_̶v̶a̶l̶u̶e̶ ̶ ̶t̶h̶e̶n̶ serialized a̶g̶a̶i̶n̶.

This step should be opt-in as it will be less efficient as what we have now.

Your proposition could also be implemented in a few lines of code by having the dumps function defined as an argument with a default value for the execute/subscribe functions for all the transports. I'll accept a PR if it comes with tests.