support run HTTP server with specific net.Listener

[Zheaoli]

We have to use Gin with a special listener in some situations.

For example, if we want to upgrade Gin server graceful, we may use the third party lib to support this feature such as tableflip

Here's some example code

upg, err := tableflip.New(tableflip.Options{})
if err != nil {
	panic(err)
}
defer upg.Stop()

go func() {
	sig := make(chan os.Signal, 1)
	signal.Notify(sig, syscall.SIGHUP)
	for range sig {
		err := upg.Upgrade()
		if err != nil {
			log.Println("Upgrade failed:", err)
			continue
		}

		log.Println("Upgrade succeeded")
	}
}()

ln, err := upg.Fds.Listen("tcp", "localhost:8080")
if err != nil {
	log.Fatalln("Can't listen:", err)
}

go router.RunListener(ln)

if err := upg.Ready(); err != nil {
	panic(err)
}
<-upg.Exit()

time.AfterFunc(30*time.Second, func() {
	os.Exit(1)
})

_ = server.Shutdown(context.Background())

[codecov]

Codecov Report

Merging #2023 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2023      +/-   ##
==========================================
+ Coverage   98.92%   98.92%   +<.01%     
==========================================
  Files          40       40              
  Lines        2224     2229       +5     
==========================================
+ Hits         2200     2205       +5     
  Misses         12       12              
  Partials       12       12

Impacted Files	Coverage Δ
gin.go	99.13% <100%> (+0.01%)	⬆️

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[Zheaoli]

is this PR work?

[appleboy]

@thinkerou Need your review.

[Zheaoli]

I have already update the debug info.

[thinkerou]

LGTM