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/server/server.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"net/http/httptest"
 
+	"github.com/99designs/gqlgen/graphql"
 	"github.com/99designs/gqlgen/graphql/handler"
 	"github.com/99designs/gqlgen/graphql/handler/transport"
 )
@@ -131,6 +132,10 @@ func (r *queryResolver) Fail(ctx context.Context) (*bool, error) {
 func RunServer() *httptest.Server {
 	gqlgenServer := handler.New(NewExecutableSchema(Config{Resolvers: &resolver{}}))
 	gqlgenServer.AddTransport(transport.POST{})
+	gqlgenServer.AroundResponses(func(ctx context.Context, next graphql.ResponseHandler) *graphql.Response {
+		graphql.RegisterExtension(ctx, "foobar", "test")
+		return next(ctx)
+	})
 	return httptest.NewServer(gqlgenServer)
 }