Add GraphQL Extensions (#184)

This enables accessing the Extensions field, as defined in the response
format: https://spec.graphql.org/October2021/#sec-Response-Format

Extensions can be enabled using configuration option use_extensions.
This will change the return parameters of the generated client
functions. Making it a breaking change if enabled.

Since extensions are untyped as defined in the spec, the Client will
return an interface of type map[string]interface{}.

internal/integration/genqlient.yaml

@@ -2,6 +2,7 @@ schema: schema.graphql
 operations: "*_test.go"
 generated: generated.go
 allow_broken_features: true
+use_extensions: true
 bindings:
   Date:
     type: time.Time