Allow the definition of a custom json serializer

[jmg-duarte]

Basically, I want to be able to do something like

Client(serializer=orjson.loads, deserializer=orjson.dumps)

The aiohttp.ClientSession allows for custom serialization so it shouldn't be an hard implementation?
https://docs.aiohttp.org/en/stable/client_reference.html

[leszekhanusz]

There was a previous discussion about this in issue #216

The main use case was custom scalars serialization and is solved with the current version of gql so it was closed, but I can see there are other uses for this.

The arguments should be set in the transports and not in the Client though.

Feel free to submit a PR with tests.

[leszekhanusz]

I looked a bit more about this issue and found out that it is actually possible right now.

you can provide a client_session_args argument to the AIOHTTPTransport with any additional argument which need to be passed to aiohttp.ClientSession, including the json_serialize arg:

transport = AIOHTTPTransport(url=url, client_session_args={"json_serialize": json.dumps})

There are still json.dumps calls elsewhere in the transport used for:

• logging
• file uploads
• appsync headers

[leszekhanusz]

In the PR #337 I modified the transport to be simplify it. It can be used like this now:

transport = AIOHTTPTransport(url=url, json_serialize=json.dumps)