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/generate_test.go

@@ -191,6 +191,10 @@ func TestGenerateWithConfig(t *testing.T) {
 			ClientGetter: "github.com/Khan/genqlient/internal/testutil.GetClientFromNowhere",
 			ContextType:  "-",
 		}},
+		{"Extensions", "", &Config{
+			Generated:  "generated.go",
+			Extensions: true,
+		}},
 	}
 
 	sourceFilename := "SimpleQuery.graphql"