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{}.

generate/config.go

@@ -27,6 +27,7 @@ type Config struct {
 	ClientGetter     string                  `yaml:"client_getter"`
 	Bindings         map[string]*TypeBinding `yaml:"bindings"`
 	StructReferences bool                    `yaml:"use_struct_references"`
+	Extensions       bool                    `yaml:"use_extensions"`
 
 	// Set to true to use features that aren't fully ready to use.
 	//