[go] Allow user overrides for generated outputs

[jimschubert]

go-gin-server previously didn't allow users to configure apiPath, serverPort, or apiVersion. This change only overwrites those values if they don't exist.

@antihax (2017/11) @bvwells (2017/12) @grokify (2018/07) @kemokemo (2018/09) @bkabrda (2019/07)

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

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[jimschubert]

Generated with:

./bin/go-gin-petstore-server.sh --additional-properties=apiPath=asdf,packageName=jimgo,serverPort=9091

apiPath and serverPort work as expected:

/*
 * 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/asdf"
	//
	sw "./asdf"
)

func main() {
	log.Printf("Server started")

	router := sw.NewRouter()

	log.Fatal(router.Run(":9091"))
}

Head of asdf/routers.go shows packageName:

package jimgo

import (
	"net/http"

	"github.com/gin-gonic/gin"
)

// Route is the information for every URI.
type Route struct {
	// Name is the name of this Route.