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{}.

example/genqlient.yaml

@@ -2,7 +2,6 @@ schema: schema.graphql
 operations:
 - genqlient.graphql
 generated: generated.go
-
 # needed since it doesn't match the directory name:
 package: main