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{}.

docs/genqlient.yaml

@@ -87,6 +87,16 @@ client_getter: "github.com/you/yourpkg.GetClient"
 # Defaults to false.
 use_struct_references: boolean
 
+# If set, generated code will have a third return parameter of type 
+# map[string]interface{}. This will contain the optional values
+# of the Extensions field send from Servers.
+# ref.: https://spec.graphql.org/October2021/#sec-Response-Format
+#
+# This can be useful for extending the GraphQL Protocol.
+#
+# Defaults to false.
+use_extensions: boolean
+
 
 # A map from GraphQL type name to Go fully-qualified type name to override
 # the Go type genqlient will use for this GraphQL type.