[Go] Add a generator for the gin framework (#1048)

* copy from go-server and add go-gin-server generator

* change the templates for the gin

* fix warnings of the golint tool

* fix the path of script

* add samples

* delete unnecessary comments (#1048)

* make the help message more appropriate (#1048)

* fix the link address format (#1048)

* minor improvement

samples/server/petstore/go-gin-api-server/main.go

@@ -0,0 +1,31 @@
+/*
+ * OpenAPI Petstore
+ *
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * API version: 1.0.0
+ * Generated by: OpenAPI Generator (https://openapi-generator.tech)
+ */
+
+package main
+
+import (
+	"log"
+
+	// WARNING!
+	// Change this to a fully-qualified import path
+	// once you place this file into your project.
+	// For example,
+	//
+	//    sw "github.com/myname/myrepo/go"
+	//
+	sw "./go"
+)
+
+func main() {
+	log.Printf("Server started")
+
+	router := sw.NewRouter()
+
+	log.Fatal(router.Run(":8080"))
+}