feat: Add Go helper functions for HttpResponse (#2622)

`builtin/external_module.go` will have a couple of helper functions
added:
```
// A helper for successful HttpResponse
//
func HttpSuccess[E any, B any](code int, body B) HttpResponse[B, E] {
  return HttpResponse[B, E]{Status: code, Body: ftl.Some(body)}
}

// A helper for HttpResponse with error
//
func HttpError[B any, E any](code int, e E) HttpResponse[B, E] {
  return HttpResponse[B, E]{Status: code, Error: ftl.Some(e)}
}
```

Fixes: #1632

backend/controller/ingress/testdata/go/httpingress/httpingress.go

@@ -103,9 +103,7 @@ type QueryParamRequest struct {
 
 //ftl:ingress http GET /queryparams
 func Query(ctx context.Context, req builtin.HttpRequest[ftl.Unit, ftl.Unit, QueryParamRequest]) (builtin.HttpResponse[string, string], error) {
-	return builtin.HttpResponse[string, string]{
-		Body: ftl.Some(req.Query.Foo.Default("No value")),
-	}, nil
+	return builtin.HttpSuccess[string](200, req.Query.Foo.Default("No value")), nil
 }
 
 //ftl:ingress http GET /html
@@ -138,7 +136,7 @@ func Int(ctx context.Context, req builtin.HttpRequest[int, ftl.Unit, ftl.Unit])
 
 //ftl:ingress http POST /float
 func Float(ctx context.Context, req builtin.HttpRequest[float64, ftl.Unit, ftl.Unit]) (builtin.HttpResponse[float64, string], error) {
-	return builtin.HttpResponse[float64, string]{Body: ftl.Some(req.Body)}, nil
+	return builtin.HttpSuccess[string](200, req.Body), nil
 }
 
 //ftl:ingress http POST /bool
@@ -148,10 +146,7 @@ func Bool(ctx context.Context, req builtin.HttpRequest[bool, ftl.Unit, ftl.Unit]
 
 //ftl:ingress http GET /error
 func Error(ctx context.Context, req builtin.HttpRequest[ftl.Unit, ftl.Unit, ftl.Unit]) (builtin.HttpResponse[ftl.Unit, string], error) {
-	return builtin.HttpResponse[ftl.Unit, string]{
-		Status: 500,
-		Error:  ftl.Some("Error from FTL"),
-	}, nil
+	return builtin.HttpError[ftl.Unit](500, "Error from FTL"), nil
 }
 
 //ftl:ingress http POST /array/string

go-runtime/compile/external-module-template/.ftl/go/modules/{{ .Module.Name }}/external_module.go.tmpl

@@ -93,3 +93,17 @@ func init() {
   )
 }
 {{- end}}
+{{- if eq .Module.Name "builtin"}}
+
+// A helper for successful HttpResponse
+//
+func HttpSuccess[E any, B any](code int, body B) HttpResponse[B, E] {
+  return HttpResponse[B, E]{Status: code, Body: ftl.Some(body)}
+}
+
+// A helper for HttpResponse with error
+//
+func HttpError[B any, E any](code int, e E) HttpResponse[B, E] {
+  return HttpResponse[B, E]{Status: code, Error: ftl.Some(e)}
+}
+{{- end}}