feat(goctl): Support gateway sample generation (#3049)

tools/goctl/gateway/gateway.tpl

@@ -0,0 +1,20 @@
+package main
+
+import (
+	"flag"
+
+	"github.com/zeromicro/go-zero/core/conf"
+	"github.com/zeromicro/go-zero/gateway"
+)
+
+var configFile = flag.String("f", "etc/gateway.yaml", "config file")
+
+func main() {
+	flag.Parse()
+
+	var c gateway.GatewayConf
+	conf.MustLoad(*configFile, &c)
+	gw := gateway.MustNewServer(c)
+	defer gw.Stop()
+	gw.Start()
+}